Collect all the information that will be needed. This includes the purchaser's name, seller's name, date, item or service description, amount of the sale and any warranty that will be given on the item or service.
Begin writing on a piece of paper the purchaser's name and address. Denote that this is the purchaser.
Write down the same information on the seller. Denote this person as the seller.
Date the document.
Describe the facts of the sale: who is purchasing what from whom and for how much.
List any warranty that the item or service carries to the purchaser from the seller.
Sign the bill of sale and have the purchaser also sign. Make a copy of the bill of sale for each person with valid signatures.