How do I correctly change the strings on my guitar?

First, turn the tuning key until the hole in the string post is in line with the nut
slot. Next, pass the string through the hole pulling it up so there's about 2" between the finger board and the string keeping the string taught at the bridge end. This should allow for about 2½ or 3 windings on the string post once you’re done. You don't want any more than that - excessive windings can cause the string to slip continuously when tuning!
Now you will pull the string toward the center of the headstock and loop it underneath and over itself up against the string post. Next, start turning the tuning key winding the string on the post. Be sure the windings go from the top down thus creating a greater angle from the string post to the nut. This puts more downward pressure in the nut and increases your sustain. By installing the string in this manner it will have wound around itself once giving it something to “hang onto”, and once properly stretched should stay in tune great!
Next we'll talk about stringing a guitar with vintage style Kluson tuners which have a split post (these are found on many Fender Strats and Telecasters). What you do is pass the string through the bridge end and pull it tight. Measure about 2" past the tuning post of the string you’re changing (I use the handle of my plastic string winder which is just about 2" long) and cut the string. Now stick the cut string end down the center hole of the post and start winding. Cutting the string like this you’ll have about 3 windings on the string post and once again be sure to wind from the top down! Properly installed strings always stay in tune better, have greater sustain and take much less stretching to become stable.