first I have a shirt that is too small for me and I cut it in half down the back.
Then I actually marked where I wanted the chains to hook through with a marker.
working in a row of 5 chains I inserted the hook from one side of the shirt to the other as if I were stringing a shoe lace into a shoe.
And working a chain as I go along.
Depending on how small the shirt is, you can add more chains but you would have to experiment with what fits best.
However in the end, you end up with a shirt that fits and has a different look in the back!