With a permanent marker, make two marks on the back of the phone case. The position of your markings is based on personal preference and how you like to hold your phone. I wanted my strap to be at a diagonal. You can also position it vertically.
Note: In retrospect, after using the strap for a few days, I would have moved the top down a bit as I tend to hold the phone on the bottom half versus the middle. If I make another (which we all know I will), I would just shorten the length of the chain and place the top screw lower. Again, everyone holds their phone differently, so try taping the chain in place before making your holes to be sure of placement!
The phone case I’m using is made of soft plastic so a leather puncher is a great tool to make small holes. If your case is made of harder plastic, you can use a rotary tool to drill a small hole. Always be sure to make a hole smaller than the screw – you can always go bigger if need be.