The first step is to cut holes in the bottom of your bag. Don’t be shy - cut a bunch, Drainage is key because you want to keep your soil moist but not wet.
I then put the bag on top of the screening. I cut around the bottom of the bag to get a piece of screen that will fit into the bottom of the bag. It doesn’t have to be exact, just big enough to cover your holes. Plastic screening is great because it keeps your soil in and lets the water out. You can also use a great product called Better Than Rocks or some newspaper or coffee filters to cover the holes.