You need a big fat tapestry needle from your sewing store! You do! We need to be able to thread that thick yarn through the needle’s eye. Then you simply thread the loose end through the eye and sew backwards and forwards under your stitches (so your thread is concealed and woven in) 3 or 4 or 5 times. Then snip the end close to your work and give the granny square a wriggle so the end disappears under your stitches!
Crochet handmade Hello Pip-Life