Hold the strings in the back and crochet over them for a few stitches until they are secure. You can also tie the old and new colors together in the back and weave ... m always looking for new ...

Berry Blue #9606 I/9 (5.5 mm) crochet hook 4 or 5 ... rather than in the stitches themselves. Bind off, then stitch up the back of the sleeve. Weave ends in. Repeat for other sleeve.