A guitar strap is a necessity if you ever intend to play the instrument while standing up. However, if the strap is set to the wrong length, it will be uncomfortable and the guitar will be difficult to play well. Your playing technique and height determine the proper length of your guitar strap. Luckily, the length of most straps can be simply adjusted by pulling a section of the strap through a buckle piece.
Slip the holes in the ends of the strap over the strap buttons at the bottom and top of the guitar body. The buckle piece should be pointed away from the guitar. Do not let the strap twist while attaching it to the guitar.
Put the strap over your shoulders and stand up. Keep one of your hands on the guitar until you are sure you have attached the strap properly.
Adjust the strap by pulling one end of it through the buckle until the guitar sits across your stomach. You should be able to comfortably rest your forearm on the body of the guitar and you should be able to fret notes and chords as though you were sitting down to play the instrument. If not, readjust the strap.
Play the guitar for a few minutes and make any needed adjustments. Playing comfort is the most important aspect of adjusting a strap. If you like the guitar closer to your chest, leave it there. It all depends on your style of play.
Try adjusting the strap while sitting down if you are having trouble finding the proper length. Once the strap is tight against your shoulder, but before it actually begins to raise the guitar off of your lap, stand up. This should place the guitar in roughly the same place as it would be if you were sitting down.