Purchase your strap from a local guitar store if you haven't already. Choose from nylon, leather or suede guitar straps. Make sure you get one that suits the color of your guitar and that you like the look.
Locate the metal, or plastic, knob on the bottom of the body of your guitar.
Attach one end of your guitar strap to the knob by slipping the buttonhole at the end of the strap over the knob. Ensure that the knob has come right through the middle of the hole so that it is secure.
Find the knob at the top of your guitar's body on the back, just bellow the base of the neck, and repeat the previous step. If you have an acoustic guitar you might not have a second knob. If this is the case, pass a piece of string, or shoelace, through the button hole on your strap, then tie it onto the very bottom of your guitar's head in between the first tuning pegs and the headnut.
Ensure the strap is secure and put it over your head so that the top end of the strap passes over you shoulder toward the guitar neck and the back end goes down your back finishing at the bottom of the guitar, by your hip. Adjust the strap if it is too long or short by using the plastic adjustment in the middle of the strap (the same as bag adjusters).