I noticed several of the pics have brands showing. These are not intended as recommendation of any particular brand or source.
Step 1: Design, Materials, Tools
I went with .065 inch steel as I didn't really need lots of strength, and it cost 1/2 as much( as 1/8th), but it was more difficult to weld and is a bit more fragile and prone to burn through when welding in the breeze outside. The final structure is very strong - easily supports me.