Get a small gravel vac, start it, shuffle it around in the gravel until half the water is gone (50%). For the 100%, scoop your betta into a cup, remove him from the tank, and gravel vac out as much water as you can, then dump the rest, refill with clean dechorinated water each water change. You do the 50% once a week (say, tuesday) and the 100% a week (say, saturday).
Edit: Be sure to vacuum the gravel each change btw, gunk builds up in it and can harm your fish with all the ammonia it'll be producing.