Take a spoon of contact cement (or less) and add it to the plastic bag. Squeeze all the air out or the cement will harden in the bag and then your stuck with a flat tire. Don't suck the air out with a straw, the contact cement is quite poisonous. Roll the bag up so it takes less space. I used a Ziploc bag and rolled and squeezed all the air out before sealing the last bit of the bag.