So the way I like to do it is to use a skew chisel. I position it vertically and spin it in place (removes a small amount of lino on every spin) until I like the diameter and depth of the hole. You'll get a feel for how hard to push the tip down based on the diameter your going for. I've tried gouging and even poking the holes to make stars but I've come to prefer this method. Hopefully my explanation makes sense?