You should have the same number of stitches in the cast on chain for each sleeve so that will make the right and left sleeves equal lengths.
"Cut the yarn. Ch 16-19-19 (25-28), crochet as before over back piece, at the end of row ch 16-19-19 (25-28)."
Cast on for one sleeve, continue by working the back sts in pattern and at the end of the row, cast on for the second sleeve. Now when you work back, you'll work over all the sts: 2nd sleeve, back, and first sleeve. You'll have a T shape with 16-19-19 (25-28) additional sts on each end depending on your size.