There are some guides showing the methods of farming female soldiers. But none of them show how to farm S++ soldiers properly.

With the help of some modding it is possible. You can fill your whole base with these soldiers. BUT it is going to take some time.

Mods are necessary to pull this method. If you modded this game before then you are probably familiar with Infinite Heaven, Snakebite etc. If you've never modded this game; it is really easy. I am not going to deeply explain the modding section. If you need any help you can find guides on Youtube.
Also i would like to note: You can use this guide for farming default S++ soldiers too (obviously).

Mods and the program you need:
-Snakebite
-Infinite Heaven
-IHHook
-Zeta*
-FultonExtraction3000©**
-S plus plus Soldier Mod (Updated for 1.15)***
*: I am not so sure if Zeta is really necessary but it is a great add-on for Infinite Heaven.
**: You need the "Infinite Heaven Version" from the rar file.
***: You need the "spp_100%" version from the rar file.
You can find all of these mods and SnakeBite in Nexusmods.

Preparation:
After setting up your mods and installing them to your game via Snakebite, it is time to prepare Infinite Heaven and your loadout.
-Return to your ACC.


-Open IH menu by holding your dash button or simply pressing F3
-Enter "16- Mother Base Menu"


-Make sure Mother Base Menu looks like the screenshot I provided. Especially focus on red ticks.

-After setting up war games option to enemy invasion, IH automatically enables helicopters and I have no idea why this happens. Disable helicopters. They will make things harder for you.

-Close the menu and prepare your loadout. You need a box. That's it. Use the water resistant one if you can because sometimes it rains. Again, making things harder on your side.

Deployment To Motherbase
-Head to the mother base. As soon as you leave the chopper, you are going to see some messages about 36 soldiers being extracted. Notice there is nobody around. This means everything is working properly.

-Now, equip your box and fast travel to another unit's plant via loading zone. For example: We begun at main plant. Now fast travel to R&D unit's plant. Then, from R&D to Medical plant. Repeat this procedure for three-four times.


-After fast traveling from one unit plant to another for three-four times, call your chopper. Get in and leave your base.
-If you get this message. YEAH YEAH WE GOOD
-Look at your brig section, you are going to see it is full of S++ female soldiers.

What Is Happening?
Thanks to IH, we manipulated the game. Game spawns female enemy soldiers to your mother base. Since they are considered enemy, if we fulton them we can use them for our MB.
If you still haven't realized: As soon as we spawn to a new area, Fulton3000 mod directly fultons all of these soldiers.

For fulton, limit is 100. That's why i said fast travel 3-4 times then return to your chopper. We activated the "more soldiers on MB plats" option to decrease the number of fast travels.
I think there are some mods out there which erases that fulton limit but I don't remember them working out for me.
No, you are not going to get banned. Just in case, make sure to disable your mods if you are going to play FOB.
Yes, it is going to take some time. I don't know if there are other alternatives.
Yes, I have other mods. That's why Snek looks like that and I have a SPAS-12 inside ACC.

Well done, you have hundreds of S++ goomah.
If you have questions, ask them. I'll do my best to help.
I checked this guide multiple times and fixed the mistakes before publishing it. Despite this there might be some mistakes here and there. Sorry about them :/