You can join the new end of yarn by just holding about a 6 inch tail in place with your left hand until you're knit a couple of sts. Later you can snug up the loose first st when you weave in the end.
Here's one way
of weaving in the tails. You can weave in vertically too by picking up the back loops of sts. Just thread the tail through a tapistry needle and weave in through several sts in one direction and then back in the return direction.