There are a few ways that you can do this depending on how many you have to do. One way is to fold each shirt and half and press and a light centerline. A quick way to do that is to use a 3 foot metal ruler. Lay the shirt flat, place the ruler on the shirt over the neck and down to the waist, and fold the shirt over the ruler. Line up the shoulder seams and the waist hem, then slide the ruler out. Once your vinyl is weeded, make a little mark on the top and bottom to mark the center. Generally for designs on the back, go down about 4 inches from the collar seam, and center it. If it's a front, go down about 3 inches from the seam. For a left chest, I generally go down about 3 inches from the neck seam, and center the design below the left edge of the collar. When it's on the table, this might seem like it's too close to the center of the shirt. But it ends up in a very good spot when the shirt is worn. This should be checked out though by having someone wear one of the shirts and place the chest design in the proper spot, tape it down and check the measurements. I usually use a plastic sign making squeegee for these measurements instead of a ruler.
Anyway, this is just one way to accomplish this.
Anyway, this is just one way to accomplish this.