Since some levers are, as described above, easier to close than open, I just take the time to adjust with that in mind, and wish the instructions were not written only for people who already know how this works!
Open it (the sling or a shop rag etc is about the best trick for "a tool" to open I'd use) then loosen the nut a lot.
Close it. Doesn't matter how tight to close during this entire process.
Check fit. If floppy on the rail, open and tighten a bit. Repeat carefully, until it's no longer wobbly on the rail.
Then, close and open, tightenting ONE click at a time, until the opening force is "firm" but not difficult to open.
Then, I usually pull it all off, lube it properly and scrupulously clean the rail (as I just cycled a new mechanism many times) and do one last check (maybe you were tightening onto a piece of dirt, now it's looser) then tighten and leave it there.