[header] How to drink beer out of a duct tape cup [title] Find a suitable glass or a cup. [step] This will be used as a mold for the duct tape cup. [substeps] Measure for the cup. 
Take the glass' circumference but just aim for making the height half of the glass' actual size (or a little shorter than it actually is). This will give the shape of the cup but not the same large size.