Purchase an imperial tape measure.

Just make sure it's got all the weird stuff marked so you can go down to 3/32 and all that jazz.
What I'm doing with the mini is convert to mm and then I cut out the cook chamber. I've bent up the walls to cut down on welding. I'll weld the top and bottom on so I have basically a complete cc. I'll then make the charcoal chute and firebox complete. Then and ONLY then I'll make the frame to fit the cc and chute assembly.
That way I can fiddle the measurements to ensure it all fits together nicely. And once it is inside the frame then I'll measure the outside of the frame and cut and bend the outside skin accordingly.
I got bit in the bum a bit with the double pan by cutting EVERYTHING out and then assembling the frame and then fitting the cc and chute. A lot of grinding, cutting and swearing came out of that shed mate... I'd like to think I am a tad wiser for all that graft.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford