To prepare for the prank eat some dairy products or beans. Anything that will make you fart.
Invite over a group of friends to have some drinks. Have someone be the designated camrea man.
This prank is the most funniest when you pick the person the with the worst temper of the group. After your target is picked make them a mixed drink with a straw. I've noticed most people will drink from the straw when it is included in the drink. Keep making them mixed drinks with straw included all night until you feel a huge fart waiting to be ripped. Then get target to leave the room without his drink by any means possible. A good chance for this is when target goes to take a piss.
Gather your friends around to watch. Take straw from targets drink and insert into your asshole about an inch up. Place targets drink on the floor then squat over it and stick the straw hanging from your ass into drink. Then fart as hard as you can. Make sure to film close up of the fart bubbles going into drink.
Act as everything is normal when target comes back. Watch him take a drink from the ass straw and fart infused drink. The whole groups laughs hysterically. Target looks around dumbfounded. Show him the footage of what you did to his drink. The group laughs even more hysterically.
Most likely you will end up with the drink in your face or punched. But it is well worth the laughs.
Last, kiss and make up with target.