I just took my shirt and did a rough line in chalk so I had a guideline of where to cut. If the shirt is too big then you can also cut down the sides and just take it in while you're at it.
Also, leave your straps thick.

Cut around your outline. I left the back neckline higher so later I'd have an easier time attaching the straps.

This is where it's important you leave your straps thick. Take your scissors and cut the straps into thirds.

Flip your shirt over and cut all three pieces where they attach. Make sure you're cutting on the back.

Flip your shirt back over and you should have three long pieces still attached to the front. You can trim it thinner where the seam is so it's not lumpy when you braid it.

Tightly braid the three pieces on each side.

And then your shirt should look like this from the front.

Then just attach the straps back to the shirt and sew any parts that look like shit. I had to clean up some parts of mine.