the ADA pans i have made are like this
fold the 3 sides only and when you reddu for the thresh hold ( the part where you walk in ) i take a piece of 1 x 1/2 screw it to the floor and over lap it with the pan material ( usualy lead but clory will do depending on your local codes ) then a roll our membrain gets stuck from the end of the pan material to the floor i would go out a foot or so and then paint the pan with a tar type paint the paint i use is callet HML-5000 i can be bought at pro lumber yard supply houses
Plumber
plumbing and construction