"Right now I just need to measure one! A good starting leash length goes up to here on your arm." She presses the bulb against his outer arm below his shoulder and holds the full length of leash along his arm, seeing how far it goes. (This amount of touching a stranger without permission would be embarrassing if she were less focused on what she was doing! But she hasn't noticed, yet.)
"Ueah, we want this to be about two bulbs longer, give me a sec..." She fiddles with the handle end for a bit, letting some rope pass through the knot assembly, making a bunch of small fiddly adjustments before she's satisfied.
After confirming the new length on his arm, she grabs the other one "It's surprisingly disorienting if they aren't the same length, so it's best to lay them out side-by-side to match them" and repeats the process.
"Okay! We can start with whichever hand you want. I think in some ways it's easier to start with your off-hand, because it means adding in the second one comes a lot more naturally, but I think people who are less coordinated find that approach more frustrating to start with so it's up to you, really."