I'm sure you could read a million posts and instructions and watch videos, but the best way to learn if by having someone physically there showing you how much pressure, and doing a hands-on demo.
That being said, I run a length up the back of the tendons, from above the knee to mid-cannon. Holding that end up, I then turn the wrap and wrap once around the top, just under the fold, always wrapping from the cannon to the tendons, so the spirals go in at the back of the leg. Once I have gone around once and the folded vertical strap is secure, I let it fold over, down the entire length of the tendons. From there I continue wrapping over the flap at a consistent, even pressure. Not too tight but not so loose that it'll fall off. When I get to the pastern, I go down under it, and scoop it up in the back, making a V in front, and head back up the leg. I try to end in the middle or towards the top of the leg. Most polo's now-a-days have velcro to secure them. Mine don't so I use bandage pins and tape over them.
DO NOT use polos in wet conditions! They will stretch out and slouch and even come off and can trip your horse. Also do not leave them on for too long, and rub your horses legs down when you take them off. I highly recommend having some one help you out hands on the first few times
Here's an easier way, but just as effective: