Hang a small piece of (uncooked) meat (bologna) inside a gallon jug. fill the jug 1/3 full of bran. Cut a hole near the top of the jug and cover with 1/2x1/2 screen so the flies can get in to lay eggs. As the maggots grow, they fall off the meat and are cleaned by the bran. You can increase the size of your operation as you see fit.