Most of the boats do as ssbol suggests. You can get a snap set tool which uses large square tip Vise Grips:
At Defender for about $20.
The cheapest is the $5 hammer set for the button/female at Home Depot:
I have a complete set of about 6 dies which go on the Vise Grip--but these are more than you need for just a few simple button/female snaps.
Avoid the cheap sheet metal pliers, with out any compound level action (as the Vise grips do) They just don't have enough force to get a good set.
I use the 3/8 size male and female "snaps". I have also put heavy duty velcro on both the cabin side above the window frame, and on the curtains. This will hold it up well in the middle. You can put another snap or to for that also. Be sure and put snaps on all bottom corners.
I poke the hole with a small soldering iron--It seals the edges better than a small hole punch. I also put a small square or circle of 8 oz Dacron to give a little more 'substance" to the set of the snap.
I recently bought 20 3/8" boat canvas snap caps, and the sockets in all stainless steel. Look at the reviews. There are some that are cheaper, but the reviews reveal that they are thin poor quality SS.
Defender has the full set: Visegrips, full set of setting dies, and 450 various snap fittings for about $75. I don't know the quality of fittings, but usually Defender has good quality. If you are going to start doing a lot of canvas work this is worth buying. I always keep a few extra snaps, cups, etc in the "spares kit", along with one of the cheap hammer die sets for emergency repairs when on the water.