How do you hide the final stitch in crochet?

I think you mean the final strand or tail left after you complete your final stitch. That strand (and any other tails left dangling in your work) should be pulled through your last stitch, then sewn through the stitches to hide it. I generally go a few stitches in one direction, down the post of a stitch, and turn and go in the opposite direction, to make sure it stays in place. Try to match color and weave such that the strand is not easily visible.

One should always leave a tail several inches in length. A friend of mine suggested that an appropriate length would be from the wrist to the top of your middle finger. which for me is about 6".