I managed to sew a pair of shorts for Colin with that XXXL shirt my brother wanted to throw away.
It was simple. Just place a pair of his shorts (folded on half) on the shirt (line the bottom of the shorts to the bottom of the shirt to reuse the hem), outline the shorts on the shirt after determining the seam allowance and upsizing if any. Cut along and sew according to tutorial.
Add my label and it's done!