Playtesting etiquette? What is that?
More seriously, there's no predefined standard procedure for this sort of thing. You just make a post or contact people, and ask whether they would be interested in testing something for you.
Then you wait for them to play your missions, and see if they uncover things you missed, then you fix those things, redistribute the missions, test some more, and when you're certain that the big issues have been found and addressed, release the things to the larger public.
Then you fix the issues found there, and release a patch.
The thing to keep in mind is that testers can and should try to do anything in their power to break missions. Go against the mission logic, try stupid stuff like jump out immediately and see what happens. Things like that.
Also keep in mind that playtesting is not only about checking whether the gameplay works, good testers will also give you feedback on your writing. Do not dismiss this feedback, as it is crucial to making your work be the best it can be.