For a square hole, yes, drill it and use a file. They make a 'nibbler tool' that cuts something like 3/32" square nibbles, starting from a small hole, but if you don't have one, the file is it.
That being said, the square grommet must be inserted from the side, so the square hole must start at an edge. It can't be in the middle, can it? Unless that grommet is split? If so then a square hole at an edge is a bit easier, 2 of the sides can be done with tin snips (better are aviation shears).
Try it in a piece of cardboard first, so you can see how to get that grommet into a hole. X-Acto will do fir that.