Measure how wide you want the sides of your trash screen to be and cut 12 boards that width (you'll use 6 on each side)
Measure how wide you want the front to be and cut 6 boards that width
Attach posts with wood screws to each side of the 6 side boards so you have two side sections with the rails evenly spaced the height that you need
Attach the front boards at the same spacing with wood screws
Move into place and you're done!