Fastening the crimp seal to a vial is best done with a special tool called a "crimper". Small shops will use hand crimpers or bench crimpers.
Each size and type of cap requires a different tool. For example, 13mm crimpers only work with 13mm seals. And a crimper for an ordinary 13mm band seal will not work with a 13mm Flip-Off cap.
Because crimpers are precision tools and not produced in large volume, they are expensive.
VialHut actively seeks used crimp tools for resale here.