The bottom is easy, I just use hammer & screwdriver like a chisel to break it out. I then use a half round file to get rid of any rough edges followed by sandpaper. The holes in the sides are a little tricky. I carefully drill a small hole in the sides and then I use a curved pair of pliers to nibble out the shape I want. I again file and sand to get rid of the rough edges. I give it a good rinse in hot water and soak it for a few days before it goes into the tank.
I do break a few, I just make sure to buy an extra so I don't have to make a trip back to the craft store.
This one broke on me but I was still able to use it.