Lay the granny squares flat in the desired arrangement
Match the edges and corners of two squares
Thread a yarn needle with matching yarn or use a crochet hook
Join the squares using whip stitch through the outer loops
Or join using mattress stitch for a nearly invisible seam
Or join using slip stitch with a crochet hook
Or join by single crocheting the squares together
Work from one corner to the next corner along the edge
Keep stitches even and not too tight
Continue joining squares row by row or in sections
Weave in all yarn ends securely
Block the finished piece if needed