the edge to make sure they're the same and your drawer is straight. i thought i took a pic of this, but i guess not and i'm sure you can picture it in your head right now, i have faith in you.
once my drawer was nice and straight and level, i put four screws to hold it in place, two on each side. before you start with the second drawer, i would suggest you place the container(s) that will house your plants in first. you may have a difficult time getting them in after. i shoved old newspaper and other packing materials in behind them, just to make sure that 