This is a ten step proceedure.
1 - Buy a gas mask from an army surplus. Buy a replacement charcoal filter for the gas mask online (the ones that come with the masks are often in poor condition).
2 - Buy several gallons of Chlorox ammonia bleach, and Windex. Also a 7lb sledgehammer, some plastic garbage bags, some tape, and a hacksaw with replacement blades.
3 - Put your purchases in a backpack and go over to your friends house for a few beers.
4 - During the evening, excuse yourself for a few minutes to go to the bathroom. While in the bathroom, put on the mask, mix (all but two cups of) the bleach and all of the windex to form mustard gas, and pick up the hammer.
5 - While your friend is choking/crying from the mustard gas, hit him several times in the head with the sledgehammer.
6 - Drag your friend to the bathroom and into the bath tub. Turn on all exhaust fans in the house to help clear the air, and open any windows at the rear of the house.
7 - Use the hack saw to cut your friend into small pieces and bag them with the plastic bags you brought. Carefully tape them shut, then double bag them and tape those bags shut.
8 - By the time you're finished this the house should be clear of gas. Remove the gas mask.
9 - Use the remaining cup of bleach to scour any areas of the house that may have blood on them.
10- Pick up the guitar, put it in it's case, and exit the house. Put your friend's body parts in your freezer. Over the next few weeks, eat the meat and run the bones through your garbage dispenser. Larger bones must be hacksawed into smaller pieces to fit properly.
EDIT - This is how I would do it. . . theoretically.