SAFETY FIRST! - Aluminum flashing is very sharp! Make sure to wear protective gloves for the first two steps!
Measure around your tree base and add 3-4" for overlap. Cut flashing with scissors.
Helpful tip: because the flashing is sold in a roll, it has a tendency to want to roll up on you. Have something heavy on hand (like a brick or heavy book) that you can set on top to hold it flat.