Step 1: Hold the antler firmly, but not too firmly, like you're trying to give a hug to a sentient tree stump.
Step 2: Choose your whittling tool wisely. A rusty ol' machete is not the best choice, unless you're going for a rustic look.
Step 3: Start small, like with a delicate little notch or a jaunty little curve. Don't just go to town, like a drunken lumberjack at a kindergarden craft fair.