Man, I love these things.
I'm carrying 22 lbs. of fresh fruit from Haymarket, in this picture.
I carried that amount of produce in shopping bags, tied and hung pannier style around my neck, once. Use panniers.
Step 1: Prime the Jugs
Cut the tops off of two five gallon jugs, with anything sharp. A Bandsaw is the fastest, but a knife or a hacksaw work too.
Step 2: Lash it to your Bike
I made a whole bunch of figure eights though the jug handles. When they're full of stuff, they hang down and stay there.
And that's it!