That is exactly my procedure.Razor blades are so flimsy that you can use a regular pair of scissors. I take about 2-3 mm off each side.
I don't know if there's a trick to it, but I make sure to take the edge off in one snip. If you use multiple snips, sometimes the shim's edge turns out slightly jagged. It's okay if they're a little wavy.
Be sure to protect your eyes accordingly. The "sliver" of the blade might "fly around" when you cut the blade.