Guitar straps must fit. If they don't fit, your hands and arms will hurt. It might look macho to hold your guitar below your waist, but that is too low and your playing will suffer. The guitar strap must lay comfortably over your shoulder, leaving the back of the guitar within easy reach. Get your strap to fit the first time you use it and you should never need to touch it again.
Hook the back of the guitar strap to the guitar's strap button on the back of the guitar. The back of the strap is the end that is doubled over on itself. The leading end of the strap is single. Hook the leading end of the strap to the strap button behind the neck of the guitar.
Put the guitar on. The back of the guitar should be centred on your hip bone. The neck of the guitar should be centred on your belly button, angling up at about 20 degrees.
Use your hands to adjust the guitar. If you can reach down and feel your hip bone with your right hand above the guitar body, your guitar is hanging too low. If you can feel your hip bone underneath the guitar body, your guitar is too high.
Lay the guitar down. If it's a typical strap, open the small metal clasp and adjust the doubled-over loop to make the strap longer or shorter. If it's a leather strap, pull the small thin strap free of the slot and insert it back into a slot higher or lower according to your needs and pull the tab end of the strap tight.
Put the guitar on. Play the guitar. Sit down while you still have the guitar on and play the guitar. Stand up. Your guitar should be in the same place when you sit or stand. Make strap adjustments accordingly.