Step 1: Materials
pair of scissors
sewing tape measure
Sharpie for marking cuts (not pictured)
needle and thread (optional)
sew-on velcro (optional)
When choosing a t-shirt, it is best to pick one that has a neck roughly the same size as your dog's. That way, you won't have to deal with cutting the neck down to fit or worry about cutting off Fido's air supply.
Step 2: Measure!
around the dog's middle (middle of chest is best)
distance from neck to tail (along the back)
distance from neck to mid-stomach
shoulder to elbow (to determine sleeve length)