This overview and walkthrough isn’t 100% spoiler-free, but you can safely use it without running into any major spoilers AND without losing track of which choice you’re at. I have cognitive difficulties, so I made something that was easy for me to understand. Hopefully it will be easy for others to follow, too!

This guide is based on the CG capture flowchart in the [Japanese] sweet pool OFFICIAL VISUAL FANBOOK and is carefully designed so that choices are as relevant to each individual route as possible. Unless the English-language version contains any major changes to the game, this walkthrough should still be accurate.

I've been working on a much more detailed version of the walkthrough which explains sweet pool's points system and shows you how to navigate the game without following a guide. You can see that guide on my Dreamwidth journal[katsue-katja-fox.dreamwidth.org] (you don't need an account to access it!). I'm hoping to code it for Steam later, but right now I need sleep really badly because I've been up the last two nights rewriting and coding the guide.

sweet pool is the property of Nitro+CHiRAL[www.nitrochiral.com] and Jast Blue[jast.blue]. I own nothing here!

The Director's Cut patch adds back the adult content that was removed for the Steam version. The patch can be purchased from Jast USA's site and is located here:
https://jastusa.com/games/blue001_dlc/sweet-pool---dlc-steam

23rd Dec '18
There are currently ongoing problems with the patch. Hopefully this will be fixed soon, but at the moment, please do not follow the instructions in the ReadMe file that comes with the patch, and instead, follow Alwaysmadowl's excellent guide carefully.

Also, after installing the patch, you might find that you are unable to get the Steam achievements for finishing the various routes. Also, I believe that at the moment, you can't get some of the other achievements (100% CG completion, amongst others) without the patch.

1st Jan
The patch was apparently updated (29th Dec or a little earlier). As soon as I get chance, I'll test the updated patch out to see what it has or hasn't fixed

If you don't already know how to play a visual novel, it's pretty straightforward. You press Start on the title screen, then you read the text and enjoy the pictures and music. To advance to the next page, you just click/tap/use keyboard shortcuts and carry on that way until you come to a choice. You then pick one of the two options and carry on reading. I'd strongly advise saving your progress so you can easily get back to where you left off. The choice screens are good points to save at. Depending on which choices you make, you will go down one of 6 routes to reach a specific ending. The aim is to finish all of the routes.

sweet pool has 6 different endings - 1 for Zenya, 1 for Makoto and 4 for Tetsuo. I’d strongly recommend using the following order:

Okinaga Zenya
Mita Makoto
Shironuma Tetsuo - Diving Deep (Tetsuo End 3)
Shironuma Tetsuo - Red Road (Tetsuo End 1)
Shironuma Tetsuo - Miracles May (violin version) (Tetsuo End 2)
Shironuma Tetsuo - Miracles May

This is the order in which the endings branch off from the main route. Zenya is first, then Makoto. After that, the first two Tetsuo endings branch off from the same place as each other. The second two Tetsuo endings branch off from (the point which becomes) the next choice along. You won’t see a choice there while playing the Tetsuo - Miracles May (violin version) ending, but the choice will appear once you have completed it, enabling you access the final ending, Tetsuo - Miracles May.

Selecting options is done by choosing either “Instinct” (red; top right corner) or “Reason” (blue; bottom left corner) - I’ve marked them on this image:

Like with many visual novels, you have a certain amount of leeway in what choices you make to get to a specific ending. Once you know how sweet pool's points system works, you can pretty much tailor this guide to your own preferences. What I've posted below, however, should enable you to see all of the CGs, reach all of the endings and not miss out on any text.

In case it isn't self-explanatory, the walkthrough tells you which choice to make at which scene. I've gone with a brief description of the scenes so as to avoid any major spoilers. If anyone knows how to colour-code these choices on Steam, please let me know - it makes the guide so much easier to follow!

Any choices that I've put in square brackets are choices you shouldn't encounter if you follow this walkthrough exactly.

Instinct - Prologue
Instinct - Zenya (with drink can)
Reason - Trash
Instinct - Hamburger
Reason - Bath
Reason - Outside chemistry lab
Reason - School roof
Instinct - Shoe lockers (with Zenya)
Instinct - Corridor (with Zenya)
Reason - School roof (with Tetsuo)
Instinct - After school (Makoto)
Instinct - Talk with Zenya?

Reason - Prologue
Reason - Zenya (with drink can)
Reason - Trash
Reason - Hamburger
Reason - Bath
Reason - Outside chemistry lab
Reason - School roof
Reason - Shoe lockers (with Zenya)
Reason - Corridor (with Zenya)
Reason - School roof (with Tetsuo)
Reason - After school (Makoto)
[Reason - Talk with Zenya?]
Reason - Fish tank
Reason - Dispute
Reason - Parting with Tetsuo
Reason - Makoto's visit
Reason - School roof (lunch break)
Reason - Nap
Reason - Chemistry lab

Instinct - Prologue
Reason - Zenya (with drink can)
Instinct - Trash
Instinct - Hamburger
Instinct - Bath
Reason - Outside chemistry lab
Reason - School roof
Reason - Shoe lockers (with Zenya)
Reason - Corridor (with Zenya)
Reason - School roof (with Tetsuo)
Instinct - After school (Makoto)
[Reason - Talk with Zenya?]
Reason - Fish tank
Instinct - Dispute
Instinct - Parting with Tetsuo
Instinct - Makoto's visit
Reason - School roof (lunch break)
Instinct - Nap
[Instinct - Chemistry lab]
Reason - Bed
Instinct - Confrontation
Reason - Voices in head
Instinct - Determination
Reason - Do you accept us?

Instinct - Prologue
Reason - Zenya (with drink can)
Instinct - Trash
Instinct - Hamburger
Instinct - Bath
Instinct - Outside chemistry lab
Instinct - School roof
Reason - Shoe lockers (with Zenya)
Reason - Corridor (with Zenya)
Instinct - School roof (with Tetsuo)
Instinct - After school (Makoto)
[Reason - Talk with Zenya?]
Instinct - Fish tank
Instinct - Dispute
Instinct - Parting with Tetsuo
Instinct - Makoto's visit
Instinct - School roof (lunch break)
Instinct - Nap
[Instinct - Chemistry lab]
Instinct - Bed
Instinct - Confrontation
Instinct - Voices in head
Instinct - Determination
Instinct - Do you accept us?

Reason - Prologue
Reason - Zenya (with drink can)
Reason - Trash
Reason - Hamburger
Reason - Bath
Reason - Outside chemistry lab
Reason - School roof
Reason - Shoe lockers (with Zenya)
Reason - Corridor (with Zenya)
Reason - School roof (with Tetsuo)
Reason - After school (Makoto)
[Reason - Talk with Zenya?]
Reason - Fish tank
Reason - Dispute
Reason - Parting with Tetsuo
Reason - Makoto's visit
Reason - School roof (lunch break)
Reason - Nap
Instinct - Chemistry lab
Reason - Bed
Reason - Confrontation
Reason - Voices in head
Reason - Determination
Reason - Do you accept us?
[Instinct - Pool]
(You won't encounter this last choice unless you've already completed this route once)

Reason - Prologue
Reason - Zenya (with drink can)
Reason - Trash
Reason - Hamburger
Reason - Bath
Reason - Outside chemistry lab
Reason - School roof
Reason - Shoe lockers (with Zenya)
Reason - Corridor (with Zenya)
Reason - School roof (with Tetsuo)
Reason - After school (Makoto)
[Reason - Talk with Zenya?]
Reason - Fish tank
Reason - Dispute
Reason - Parting with Tetsuo
Reason - Makoto's visit
Reason - School roof (lunch break)
Reason - Nap
Instinct - Chemistry lab
Reason - Bed
Reason - Confrontation
Reason - Voices in head
Reason - Determination
Reason - Do you accept us?
Reason - Pool

This will get you all 6 achievements for reaching each of the endings, and will also give you an extra achievement for unlocking the final (Miracles May) ending. To get the other achievements, follow Kimchi Tea's guide, linked below.

Rather than duplicate Kimchi Tea's really handy guide, I'll link it here:
sweet pool achievement guide - by Kimchi Tea

At the time of writing this (1st Jan '19), there still seem to be some issues getting all the achievements without the Director's Cut (aka NSFW, 18+, Adult Content) patch.

This walkthrough is a reworked, and simplified version of the guide I created in May 2017. As I mentioned at the beginning of the guide, this version is based on the CG capture flowchart in the [Japanese] sweet pool OFFICIAL VISUAL FANBOOK. When I wrote the original, I didn't yet have a copy of the Official Visual Fanbook, so I pulled together information from a number of other guides .

These are the walkthroughs I consulted when putting my original guide together. Click on the following links at your own risk - some of them may be on blogs with NSFW content and others may contain major spoilers or lead to blogs that can't be accessed without a Tumblr account:

fyeahsweetpool[fyeahsweetpool.tumblr.com]- contains spoilers and a lot of useful info about the points system
Japanese walkthrough[half-a.net] - no spoilers
blpaca - no spoilers
YaoiOtaku - no spoilers
chem-is-amani - spoilers

Other walkthroughs that I found after I'd pulled mine together:

NITRO+CHIRAL PATHS[nitrochi-paths.tumblr.com] - spoilers
diving deep[rahenna.com] - no spoilers, but the page has sadly gone

I'm sure there are other walkthroughs online that I haven't come across - I'm not deliberately leaving anyone out!