You have 2 options. Option 1: buy a pre-trimmed shell. Option 2: Trim the shell by yourself. I have a pre-trimmed shell as it's a very critical step. This guide is meant for pre-trimmed shells only. If you need to trim your own shell, please check on Google what needs to be trimmed and make sure it's trimmed perfectly.

Put the double side tape in the shell as shown on photo 2. Make sure it's good at it's place.