Its odvious that to allow someone to tie to up requires allot of trust because you will be completely defenceless against them and the rest of the world.
You are dependent on them to protect you if anything happens,
to let you go and to play nice however you like it,
always tell them you limits you will both have more fun that way because if you enjoy, it'll happen again.
Also just talking about what can and can't happen with you will help you get an idea whether or not you want a person to play with you or not and allow them gain your trust and you gain their respect and if it doesn't feel right just keep talking and you still gain a friend.
Again that's right "can't happen" apply's here, just remember even as a submissive you and your wishes should be respected unless you choose to give it up.
This is something that we do to have fun and so if you don't have fun don't do it. Trust is a big part of that fun, you need to know that you have nothing to worry about and then you can just relax and let you and your partner play.
Plus when your locked up trust really is the key to set you free ;)