To perform band pulldowns with a barbell, take two equal bands and suspend them from the top of the squat rack like you would a reverse band bench press. Suspend a barbell in the bands and tighten/loosen the corresponding bands to get the bar to sit straight across the bands and perfectly parallel to the floor. Sit down below the barbell and reach for the bar and pull the barbell down. Pulling with the elbows and focusing on the contraction of the muscles in the upper back will allow you to get the most out of this exercise. Since you are using two bands for resistance in this exercise you maybe finding yourself floating up off of the floor. To remedy this have somebody lay a plate or dumbbell on your lap while doing this. If that doesn't work too well you can take another barbell and lay it across the bottom of the rack and take the safety pins and set them at about the level of your knees while slightly bent. Take that second barbell and place it underneath the safety pins and the barbell will act like a knee pad on a normal lat pulldown machine.
Doing lat pulldowns like this allows you to use roughly twice as much resistance as using just one band and can keep you down anchored to the floor.
Bands Barbell Pulldowns Lat Exercise - Start Position
Bands Barbell Pulldowns Lat Exercise - Finish Position