Check whether the shoe already has an insole that can be removed, and how it will affect the installation of a new insole. A removable insole is normally easy to identify because it tends to be thick and not glued down. Because it might be difficult to wear two padded insoles in one shoe. 

Step 1: Remove factory insole

Buy an insole close to your standard size. If your shoe size is a half size, round up to the next full size. For example, if the shoe size is a 7.5, choose a size 8 insole. Larger insoles can be trimmed to fit a slightly smaller size.

Step 2: Tracing

Put the original insole on top of the new insole. Line up the back of the heel and the arch as best you can. 

While holding the original insole aligned against the new insole, use the marking pen to trace around the original insole to get an outline to cut along.

Step 3: Trimming

Cut your new insole just inside the line you just drew. Once the insoles are trimmed, they should fit properly into your shoes.