I don't actually have an Iphone so I made a cardboard stand-in. I found the dimensions at the Apple website
. Since I don't know which model the gift recipient has I made the stand-in based on the size of the largest one.
I used this cardboard stand-in (covered in duct tape) to measure the length and width I would need for the felt inside lining.
The width of the felt is the width of the phone plus twice the phone's depth (since we are making a sleeve for the phone to fit inside) and and the seam allowance.
The length is measured the same way, the size includes the length of the phone plus twice the depth and the seam allowance.
Once the felt piece has been cut out simply use that as the template to cut the same size piece of plush fabric.