Step 1: Layout and cut the pieces
we decided to pick up a power-shear from harbor freight. cuts the sheet like butter and leaves a cleaner edge than the hand sheers. it wouldn't work for really detailed work but for reasonably smooth curves it's definitely worth it's weight and even for straight cuts it's significantly more affordable than a floor shear.
Step 2: Fold and rivet the sides
remember that sheet metal is sharp. wear gloves, but don't be afraid to have fun.