I like to use a spoke shave, but it can be a bear. The blade needs to be really sharp. Most come already ground but need to be honed on a 1000 water stone. A oil stone will work, need to do the flat side first and make sure there are no grind marks left on the blade, then do the angle, a jig to hold the blade while you sharpen helps.

When sharp, set the blade shallow and move it slowly until it just starts to bite. I like to push mine, some like to pull it. Try and keep the frame flat to the surface and hold a slight angle in relation to the direction of the cut. The easiest cut to make is triming the stem, a concave place on the hull is the hardest in my book. I put the blade in with the angle facing down, I believe.