So while this is specifically about a model brain, I think a lot of the tips will still help people when making models of anything for school projects and stuff. And the first tip is actually...
to NOT USE Play-Doh. I used it because it was there, and I didn't know any better. As Play-Doh dries, it starts to crack or fall apart very easily. There are a ton of recipes to make your own modeling clay at home with basic kitchen ingredients. Some of the recipes will show you how to make clay that you can even bake and harden! So use those instead.
Another tip is to make most of the model out of cardboard, and then just cover it in Play-Doh or clay (I'm just going to use "clay" from now on).
Doing it this way saves a lot of clay, which could be crucial if you forget to do a project and have limited supplies. But more importantly, this lets you draw the different parts first, so that it's much less likely that you'll mess up.
Although...it IS still possible to mess up (see picture below), but still, it's better than messing up with your clay, since you might not be able to "un-mix" the different colors and might have to start over.
And this is what both halves of the brain look like when they're done. Surprisingly, I learned that the average brain is actually about that size, maybe slightly bigger. I always thought they were way bigger since some people have huge heads.
And remember to label all the parts! UNLESS you're making it for your sibling because they're paying you. In that case, make sure they label it so it's in their handwriting :)
By the way, the brain is actually very complex, so if you are getting paid, make sure you charge a good amount.
I hope this helps you all with your projects--thanks for reading!