Do you build the base kit before or after the shed?

Before. Assemble the base frame and get it level first, lay in the floor you have chosen, and only then build the shed on top of it. That order matters because the frame is what holds the walls square while you assemble them — doing it afterwards is not an option.

Each kit states its own frame size, so you can mark out and prepare the site before the shed even arrives: the kit for 8 x 6, 10 x 6 and 4 x 10 ft sheds frames 120.06 x 67.81 in., and the kit for 10 x 12, 10 x 13 and 10 x 14 ft sheds frames 120.06 x 142.31 in.