Rendering normal maps in Blender

I had a problem. My friend wanted me to import her Sims 4 character into Tower Unite. Unfortunately, the model had too many vertices. In particular, there's a necklace that has roughly 3000 vertices, putting us just over the 30,000 vertex limit.

Simply repairing or reconstructing the necklace would take far too long. I've got things to do.

Wouldn't it be nice if we could quickly reduce that to, say, 8?

Well, the good news is, we absolutely can. Grabbing the necklace, splitting it from the model, and rendering it out as a flat texture is actually pretty easy.

The bad news is, it's not going to look very good. It'll be flat, and it won't have any lighting information.

Normal maps solve this problem. They're a type of texture that stores lighting information in a way that can be applied to a flat texture. Ideally, we'd have the illusion of having the original necklace, but with far fewer vertices.