One major point and a few finer ones...
1) Use three or more heavy duty snaps in addition to any #15 0r #16 snaps. Otherwise, the crotch might come open in public.
2) The flap should be front over back, contrary to what is shown in the
sketch..
3) Key the crotch. Have you ever put button #4 into buttonhole #5 when buttoning a shirt, and not realized it until you had nearly finished buttoning? If you put all of the male snaps on one side and all the female snaps on the other, then if they are offset when putting them on, you won't know until you reach the end. For example, if you start in the middle, and put male #5 into female #6. This can be avoided by 'keying' or alternating the gender of the snaps. When snapping your own crotch, you won't have a good view of what you are doing.
4) Put the elastic in the waistband before the first washing if you want the familiar elastic-waistband fade pattern.