First, I pulled out an insert I already had--a non-perfected one from one of my first pairs of training pants. But, if you want to measure, then you can measure the width of the crotch of the training pants, and then make it the length you want.
Then, I cut one, long strip of fabric that was the length I want, and the width multiplied by six. If you do not have a strip of fabric long enough, you can cut out two pieces that are the length of the insert and the width multiplied by three.
And if you're doing velcro, you might want more than just two. You might want to consider doing a lenghwise strip of velcro, or just three or four little squares. THat's up to you.