I just purchased two new Silhouette mats and I kept two of my old mats. One I was going to keep to use glitter paper. The other one I decided to clean and get all the stickiness off and use as my grid to line up designs.
This works perfectly I can tell where the center is and I can make it equal on all sides and top and bottom.
This works for any shirt, you can see where the center is and you can place your design down and then see if it is equal on all sides. If not you can shift it to get it balanced. This will work when putting a design on the side of the shirt, you will be able with the grid to see just how it lines up.
I have two photos one is of an adult shirt and the other is a 3-6 months onesis and you can see how it works to line it up.