What is the best way to create creases onto a paper mache model of the human brain?

There's a few things you can do. If you use a small enough core you can use "Great Stuff" expandable spray foam to make the creases then cover them. (If you can't find great stuff any foam will work. You can get it at Wal-Mart, Lowes, Home Depot, places like that. Just use a small amount and move the can quickly because you'll have too much in one area and it will grow out thicker. If it does you can cut it down and sand it to reshape.

You could make it thick enough and cut them back in with a Dremel Tool but it probably wouldn't give the look you want.

The easiest would be just to paint the details in and leave it smooth.

Good luck

Copy a picture of a brain from your computer. If necessary enlarge it. Glue it to the mask