To get this achievement, you must win a war where each side has 1 million casualties. Losing the war and white peaces do not count.

This achievement is pretty rare (partially because it's only existed since version 1.8) but also because it's nearly impossible to get before the late 1700s or early 1800s. Armies simply aren't large enough for 1 million casualties on each side until late in the game. Judging by how rare the Just a Little Patience achievement is, most players don't play until the early 1800s.

There's no step by step guide to get this achievement, because there are literally hundreds of ways that it could be done. I will give you some general guidelines though.

If you're going to get big and powerful enough to get this achievement, you might as well pick up some other achievements along the way. Some suggestions are:
* WC / Three Mountains
* WC / One Faith / A Kaiser Not Just In Name
* Baltic Crusader
* Pyramid of Skulls / Manchurian Candidate
* The Spice Must Flow
* Mare Nostrum
* Master of India
* Big Blue Blob
* Poland Can Into Space

The point I'm trying to make is, there are many ways to go about getting this achievement, none of them are quick, but you can fit this achievement nicely into your other achievement runs.

You'll need to be prepared for a war that could last more than a decade and result in over a million casaulties. This will be easier to do if you have the following:

A. Great Economy
You'll need lots of money for replacing casualties, or for hiring mercs. Also it doesn't hurt to spend money building lots of buildings that specifically raise force limit and manpower.

B. Very High Force Limit
This war will be easier if you have overwhelming numbers.

C. Very High Army Recovery Speed
You can shrug off casualties more easily if you replentish regiments after battle.

D. Lots of Monarch Points to Spend.
You may need to reduce war exhaustion, or if rebellions get out of hand use harsh treatment or boost stability.

Here are the idea groups I used. The Ottoman Traditions gave me increased recovery speed and force limit, while Aristocratic and Quantity Idea Groups gave me higher manpower, force limit, recovery speed, and lower attrition.

By the late 1700s or turn of the 19th century, you should be on the lookout for a target. Preferably a very large nation with lots of allies. Don't wait any later than 1800 or so, since the war can last 15 or 20 years.

I'd suggest using the Imperialism Casus Belli (which you can get at Diplo Tech 23). Next, fight your war like you would any other, but make sure you win the war. You can only get the achievement by winning. It doesn't matter how much you win by. In my playthrough, a war that lasted 15 years and had 3.2 million dead resulted in me winning a mere 100 ducats.

So to sum this all up:

1. Pick another achievement run that would involve playing until the late 1700s or early 1800s.
2. Make sure your economy is good, force limit is high, manpower is high, etc
3. Be on the lookout for a very large target - you'll know it when you see it.
4. Fight a very long war, make sure that each side has 1 million dead, and take any kind of peace deal that involves you winning.