needle and thread (or sewing machine)
paper + pen (a fabric pen too if you have one)
Once you’ve flipped it inside out, your fabric will be all crushed. Give it a quick iron to get the kinks out and take a moment to admire your work. Lovely!
Position the top fastener over the mark you made and piece the snap fastener together. Once you have one part in place you can measure where to put the back of the fastener on the other side so they match up. Pop that part in and you’re done!