That's a pretty easy job, and they're less than 50 bucks over the net.

Before you apply the mastic, do a dry run so you can see how far it needs to be tucked in at the front, how the sides and back need to line up, etc. You'll need maybe a dozen sticks of various lengths to prop it up (I happened to have a bunch if 1 x 2's lying around). Mark exactly where you want it to fit at the front.

You don't need mastic where it tucks in. When you get the mastic on, start by positioning the front exactly, and everythig else will go where it's supposed to. Just leave it propped up with the sticks for a few hours to dry. The more sticks, the better, because it'll try to droop down.