I may be a little too late for this post, but I'll say it anyway. What I do is float the bag in the tank for about 20 minutes. Then I carefully dump the water and fish into a small, clean container. Then all you need is an aquarium siphon. You tie a few knots in the tubing and get a siphon going (put the end in a different container to avoid messes). Then, tighten the knots until the water is slowly dripping out. Place the end of the siphon into the container with the fish, and wait until the amount of water doubles. Then, gently scoop out half of the water, and wait until the level doubles again. At this point you are ready to catch the fish and put them into the tank. With other fish, I would just net them, but since bettas' fins are really delicate (and their will be only one fish per tank) I would use a cup and scoop them out, then pour them gently into the tank.
I used to just float the bag and then dump them in (this was before I knew anything about aquariums), and the fish would usually hide and breathe rapidly for a while. Now I do it this way, and when I put my fish in the tank they just swim around like they own the place! :D
Oh, and remember to keep the tank lights off, and the lights in the room off as well. This reduces stress and will help them acclimate faster. A little natural sunlight is fine though, the lighting just has to be subdued, not pitch black.
I really have to recommend that you keep your bettas in at least 2.5 gallon containers, it will help them be much healthier. Also, are you planning on getting heaters? Bettas are tropical fish and really need a stable temperature of around 78 degrees F.