Install the feet is very easy. Just follow the following steps.

1. Mark the bag feet placement on the bag bottoms's exterior.

2. Use plain awl, which cuts a circle that is a bit less than 1/4 inch in diameter, place the bag on a rotary cutting mat and poke or cut through the bottom (all layers of the bag bottom).

3. Insert a foot through the bag bottom. Use a small piece of stabilizer or a firm interfacing underneath the prongs to hold the foot in place. It works just like a washer. 

Note: Since the weight of the bag forces the feet into the bag bottom and basically keeps the feet in place, you can secure each foot through the bag bottom with any firm interfacing or stabilizer. Metal washer are not necessary, usually being used to stop a piece of hardware (locks, magnetic snaps, connectors) being pulled out if the force is in that direction.