Good question, lol. It wasn't easy
If I was a shorter guy than my 6'3" it would have been even harder.
She was very docile after I got my hand on her jaw, so I got underneath her so she was laying flat on my belly and legs, tail fork touching the top of my boot, nose on my chest. Laid my rod next to her with the butt also touching the top of my boot. Her nose was exactly one palm width from my first guide. I measured that distance on the rod after I got back to the truck.
I've done this before with other fish, and am confident that the measurement was as accurate as if she were lying on the beach. If anything, the slight bow in her tail would have caused her to measure a bit shorter than she actually was, but that was good enough for me.