Having worn them before at a starwars larp in the uk:
Get your eyes checked - make sure they are a normal shape and size so that you can wear party lenses.
When you get them make sure you throughly wash your hands before trying them on and buy a decent sized container of seline solution - helps to keep them clean and to get them in.
If you plan on wearing them for more than 12 hours build up to it. For example wear them for 4 hours then 8 then 12 over 3 days to get your eyes used to the sensation.
Take them out once every 18 hours for about 20mins - give them a wash and let them clean in their container meanwhile rest your eyes by looking at somthing at a resonable distance - like tree tops. This puts minimum stress on your eyes and allows them to breath. Eyes do require alot of oxygen to function and some of this is absorbed through the front of the eye. It’s a good idea therefore to let them ‘breathe’.
If at any time the lenses hurt or feel itchy then take them out - wash them and your eye then put them back in. Even the smallest spec of dust or fluff can make them itch or feel uncomfortable. They really should not feel uncomfortable to wear so make sure you douse them in seline solution before putting them in to wash off any fluff or dust that may have settled on them while their container was open.
Most important:
Your eyes are very delicate - if for any reason they become irritated or your eyes start to feel itchy / dry / tired then take them out! Let your eyes rest for about 20mins then try putting them back in again. If things are still not feeling right then just stop and find somone who wears contacts and see if you are doing anything wrong (their might be a split in them etc).
DO NOT STRUGGLE ON!
-Hope this helps