I love the convenience of a center-pull ball of yarn as you're knitting or crocheting, but don't have a handy-dandy ball winder to make that happen. So, I use this simple method to wind up my yarn by hand.
Step 1: Gather Your Materials.
Gather your materials. With this method, all you need is the yarn you want to wind up and some kind of rod or stick. I have a short wooden dowel rod that I use, but knitting needles work great too.
Step 2: Tie a Slip-knot and Attach the Yarn to the Rod.
Tie a slip-knot near one end of the yarn (if you need help with this, I recommend JaimeMarie's instructable here). Then, slide the knot onto the stick and gently tighten it.
Step 3: Start Winding.
Holding the end of the yarn down with one hand (I'm right-handed, so I use my left hand for this), begin winding the yarn on the dowel or knitting needle. Keep in mind with this step (and all other steps too) it doesn't have to be perfect!
Step 4: Keep Winding.
Continue winding the yarn onto the dowel rod, over the top of the first coating of yarn you created in the previous step. Eventually you're going to twist all of your yarn into a ball on the dowel rod.
Step 5: More Winding.
Keep winding until all of your yarn is wound onto the dowel rod or knitting needle. I tend to wind in a figure-8 pattern to keep the ball circular, but there isn't really a right way to do this. Just make sure that you keep the end that you tied to the rod out of the middle of the ball. The easiest way for me is to just hold the yarn end against the dowel rod with my left hand and wind the yarn with my right.
Step 6: Slide the Rod Out of the Ball of Yarn.
Once all the yarn is wound up, I like to tuck the loose end (not the one you tied to the dowel rod) under one of the other strands of yarn, just to keep it tidy. However, this isn't necessary.
Now you just grab onto the ball of yarn with one hand and pull the dowel rod out of the ball of yarn, pulling in the opposite direction as the end of the yarn that is tied to the dowel rod. Just pull the slip knot out of the end of the yarn, and you're done! Now you have nicely wound center-pull ball of yarn, sans fancy winder machine.