This is an easy alteration and would work on an adult or kid-size t-shirt. It's a great way to give an old, tired shirt new energy!
Step 1: What you'll need
two t-shirts, one long-sleeved t-shirt OR one t-shirt and some knit fabric; sewing machine; scissors; thread to match shirt and safety pins
Step 2: Cutting the shirt
Lay the shirt out flat. Draw a line diagonally from the center of the back of the neck to just under the arm on both sides. Cut along the line through both layers.
Draw a line straight across the top, just under the neck. Cut through both layers.
Cut about 1 1/2" off the bottom. Shirts can vary, so alter this to meet your needs. You'll be adding about 3" back on later.
Step 3: Cutting the second fabric
Cut three pieces from your second knit (second shirt, sleeves or knit fabric), each 40" long and 2" wide. If you don't have pieces that long, sew shorter pieces together, preferrably with a diagonal seam to cut down on bulk.
Cut two pieces, each 1/2" longer than your shirt is wide (for my shirt it was 18") and 3 1/2" wide.
Step 4: Making the braid straps
Stitch the three 40" pieces together at one end. The fabric I used had a definite wrong side, so I folded them in half first. If you're using t-shirt scraps, that shouldn't be necessary.
Braid together, keeping the wrong side turned in, if you have one. Pin the finishing end with a safety pin so it doesn't come unbraided.
Step 5: Armholes and neck
Hem the armholes by turning under 5/8', turning the raw edge inside that, pressing and stitching with a narrow zigzag stitch.
Turn the upper edge at the neck under 1 1/4", then turn the raw edge in 1/4". Stitch with a narrow zigzag to form a casing. Make sure to stitch all the way to the armhole instead of stopping at the edge of the casing inside.