You can do it yourself with a staple gun, course-grid sandpaper and some sort of knife. I did mine on my race bike, and it took about an hour. You can even pick up a spare seat on eBay for less than $50 usually if you don't want to hack up your stock one.
There's no standard, it's up to what shape you want for the shape of the seat. I took almost all of the foam out of mine and just left a thin layer about an inch thick everywhere. I didn't care what mine looked like since it's on a race bike, but you could spend more time getting the foam perfectly smooth with the sandpaper and cutting very carefully. I took all of that extra foam off that makes the seat "square" and molded it to be the shape of the seat pan so it's more contoured. Easier to hang off the bike for me since I have short legs.