You don't really need to practice -it takes a while to file and the change is slow and small. I start one side, file at an angle and then the other side to take a couple millimeters off at a time and use an old/destroyed negative to measure how much is enough for me. Then I file both together so that they're flush. Make sure you wash your carrier, wipe it down well and use some emery cloth to take out the burrs and sharp edges that can scratch your negative. Some tape on each side of the holes to keep the negative off the edges would help too and help clamp your negative a little better.