I usually use 3 strands of yarn, because once they are folded over, you have 6 strands, which means you can divide them into 3 lots of 2 for even braiding. I'm using my dainty ear flap hat (previous post) as an example.
Fold your yarn in half evenly so you have a loop
Insert your hook from back to front in the stitch/gap/section you want to add your tie to.
Take the LOOP and pull it through your work with the hook so it comes out the back side.
Lift up your loop and make sure you have the centre of it.
Pull the other end of your yarn (the 6 loose strands) through the loop.
Give it a tug to tighten the knot!
That's it! You have just tied on your yarn to make a braid. Now all you need to do is braid (plait) the yarn and tie a knot in the end.
I generally cut my yarn longer then it needs to be so that I have plenty of space for adding the knot at the end and trimming so they are both the same length.