This tip comes from the Tools and Tips page in the tutorials section at the Stitching Cards website.
To stop the pricking pattern from moving whilst pricking it out, secure it with two pieces of adhesive tape. These are stuck on the back of the card so the front is not marked. Put the card face down and add strips of tape to the top and bottom edges with half of the tape overhanging. Turn the card face up so the stickey side of the overhanging tape is exposed. Trim the pricking pattern leaving a margin at the top and bottom. Position the pricking pattern and press down to secure with the strips of tape.