@BeesKnees578 I pipe about 2 tbsp buttercream on the cupcake (doesn't have to be a dome shape as the sugarpaste will mould it into a dome once it's on) and then ease a disc of sugarpaste down over it! The disc should be just slightly bigger than the diameter of the top of the cupcake and about 4mm thick. If the disc is too thin it will just crinkle and won't form a proper dome :) HTH
So cute!