A sponge pre-filter is beneficial to your tank, as well as your betta, for a many reasons:
Sponge pre-filters are quick and easy to make, and are an important part of your betta tank.
1. Start out with your block of filter foam
2. Cut out a rectangle of foam that is a bit larger than your filter intake
3. Now you will cut out a channel in the centre of the foam, in which the filter intake tube will sit
They to cut a hole in the centre of the sponge that is a bit smaller than the intake tube itself, so that it's a snug fit and will stay on. (the hole outlined in this picture is large enough to illustrate where to cut, but too large for the filter tube!)
4. Reach into the space that you have cut in the block and pull out the excess bits of foam.
5. Place the filter intake tube into the hole that you've created in your sponge.
6. Trim down the edges and corners of the sponge so that it more closely fits the intake tube (and looks a bit better).
Rinse the foam and the intake tube thoroughly, replace in your filter and you're done!
As noted above, this sponge pre-filter will catch debris that is floating in your tank. If not removed regularly, this debris will rot and contribute to ammonia levels. Your sponge pre-filter should be removed and rinsed in dechlorinated tap water monthly to prevent build-up.