In this instructable, I will describe how you can make a t-shirt for your dog out of an old human-sized tee.

Step 1: Materials

The first thing you'll need to do is gather your materials. You will need the following:
dog
old t-shirt
pair of scissors
sewing tape measure
fabric glue
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!

Using your tape measure, get the following measurements:
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)

Step 3: Cut #1

Decide which side of the shirt you want showing and turn that face up on your cutting surface. Your first cut will be from the neck of the shirt to the length of the dog from neck to tail. Make your mark (and make sure you leave yourself an extra 1/4 inch or so for a hem) and cut across the bottom of the shirt.

Step 4: Cut #2

Next, you'll want to cut away the extra fabric from the sides so the shirt will fit right. To do this, mark out a section at the bottom of the shirt that is half as long as the distance around Fido's middle. Make sure that this section is centered. Then start drawing a line from your marks upward. When you get near the armpit area, use a French curve or plate or some other rounded edge to continue the line around the armpit and mark off about half of the sleeve to be cut. (If your dog has particularly thick forelegs, you may need to save more of the sleeve.) Once the marks are made, just cut along the lines. Don't forget to leave room for a hem.

Step 5: Cut #3

Turn the shirt over so that you have the stomach side facing upward. Measure from the neck down to the length from the dog's neck to mid-stomach and cut across at that length. Be sure to leave yourself room for a hem.

Step 6: Cut #4 (optional)

Depending on the length of your dog's forelegs, you may need to cut the sleeves. Go ahead and do that now if you need to.

Step 7: Glue!

Turn the shirt inside out and use the fabric glue to hem up the cut edges. To do this, put some glue down on the fabric a little ways away from the edge and then fold the edge over the glue so that the cut edge is now on the inside of the shirt. press the fabric and glue together until they set enough to stick together on their own. If you're having trouble getting some to stay together on their own, try pinning them together until they stick. You may have to do this in stages so that the glue will have time to dry. Also, for reasons that will become clear in the next step, it's not necessary to put a hem on the left and right edges of the stomach side (but don't forget to put one along the bottom edge.

If you prefer, you can use the needle and thread for this, or a sewing machine if you have one. I would just make a mess if I tried that, so I went with the much safer route of the fabric glue.

Step 8: Glue Some More!

The trickiest bit of gluing is getting the sides of the shirt that go in the armpit glued back together. With the shirt inside out and the stomach side (the short one) facing up, fold the edge of the back side (the long one) over and glue the stomach side to it.

Step 9: After Everything Dries...

Your shirt should look something like this.

Depending on the size of your dog and how much they like to be dressed, you may find it easier to make a cut down the middle of the stomach side of the shirt and sew in some velcro tabs. You could even use buttons or snaps, but velcro seems to be the best balance of affordability, effectiveness and ease of application.