Cut the leader according to the diagram on the inside of the bottom plate, and prep a few extra rolls to have in your pocket or kit-bag. I use a pair of curved cuticle scissors ( Revlon, from the drugstore ) like you'd find in a manicure set. These stay in my kit bag too.
I don't like pulling the lens to load film; I've tried it, it works, but none of the Leica literature describe/recommend it, and the Barnack is fiddly enough as-is for loading w/o taking the lens off too...
This is another plug for the Leica ever-ready case around your neck ( I use the bottom half of a IIIg case with a modern wide-strap): it holds the camera body upside down while you're manipulating the spool and cassette for re-loading.
The biggest issue I had with loading my first Barnack was with the take-up spool: the spring tab was so tight, I could not slide the leader under it: the film would just buckle-up. I swapped it with the spool from another Barnack, which loads much more easily.
One caution when trimming the leader: do NOT cut through sprocket-holes (cut in bewteen), and avoid sharp angles in your tab - make a gentle curve. Right-angles / notches / splits can catch on the edge of the film-gate and split the film as you wind it on, causing one part to wind onto the take-up spool, and the other part to get wound into the shutter curtain.
( Had this happen to a black III. )
Once you get the hang of it, it's pretty straight-forward; not necesarily "easy", but easily learnable. Pick a quiet place to practice , away from people & pets... reloading "on your feet" is more challenging, save that for once you've got some comfort with the process.
My desire to use Barnack cameras overcame the loading challenge a long time ago.
Good luck !
Luddite Frank