Measure the length of the pipe you would like to heat with the tape. Add to this the distance from the pipe to the nearest wall outlet.
Buy a length of heat tape that corresponds roughly to the combined length that you measured. Some types of heat tape run along the pipe; others wrap around it. Consult the manufacturer's guidelines to determine how much tape you will need.
Cut the tape to the precise length. Cutting can be a delicate process. Often the tape consists of heating stripes grouped at intervals. To avoid a fire hazard, be sure not to cut through a heating stripe but rather through the space between groups.
Seal any exposed bus bar with electrical tape. The bus bar is the conductive strip of metal running along either side of the tape.