What to do with tail of yarn to hide it?
I usually find that the simplest way of hiding yarn is as follows:

Cut your yarn about two inches (5 cm) away from your last stitch. Pull the yarn through an inconspicuous corner and tie an knot on itself. Weave in the end (Tie a knot or two more if it feels insecure).

If you are looking for a more secure finish:

Cut your yarn about an inch and a half to two inches away from your last stitch. Split the yarn into two strands, push them through a gap and around a stitch, one strand in front and one in the back, then tie a strong knot and weave in the ends.