Def before. I guess if your just doing liquid eyeliner you can do the lashes first and use the liner to blend the eyelash strip and make it look more natural, but if your using shadow, def use it before. The shadow is going to fall on your lashes and I would think it would be harder to do your makeup with the lashes on. I understand where you're coming from though bc I never used to use lashes bc I was too afraid of spending all that time and effort and it being ruined by me trying to put falsies on.
The best advice I can give is to just practice at home when you have time. You don't have to get all glammed out, but I find it best if you do some liquid eyeliner and then put the lashes on. That way you'll see where the lashes need to be placed when you have full makeup, but if you don't really use liner and just do shadow, then I guess you could just do it on a bare eye.
Everyone has a different method on how to apply lashes, so def practice and see what works best for you. They really intimidated me in the beginning, but I just kep practicing and now I find that using false lashes is actually quicker for me than doing my usual 2-3 coats of mascara. HTH!