Buy an 8 oz bottle of robitussin. Check the list of active ingredients to make sure the only one is dextromethorphan, also known as DXM. If it is, take the lid off of the bottle and slowly tip it backwards, with the opening resting against your bottom lip. This should help to ensure that the robitussin goes in your mouth and you don't dribble any down your chin. Drink the entire bottle. Throw up in an hour. Enjoy being 'fucked up." of the next 5-9 hours.

Dick did a better job explaining it in the link I gave you. But, now you've got the basics, since it appears that's all you were interested in.