Tying on is very similar to tying off. First, start from a bead that is a few rows back from where you are beading, leave a tail of thread extending from this bead.
Then as we did for tying off, weave through the beads changing directions a couple of times until the needle and thread is extending from the next bead in previous row, ready to continue your row.
Give your thread a little tug, if it doesn't move you are ready to continue - if it does add your needle onto the tail of thread and weave it in a little bit more until the thread doesn't move when you tug it.
When you are happy snip off the tail of thread and continue beading!