We just lept straight in, rented, and worked it out as we went.
Then again, we broke all the 'rules', mixed finances and all that 'wrong' stuff.
But we would both have been happy to just walk away if it had not worked out. We hadn't over invested in it - we moved in together, rather than co-habited, iyswim.
I am not sure how you could go about rules and regulations prior to moving in.. maybe who pays what share of the bills. But that is where we went wrong... we just pooled our money and made it last. A hideous mistake, apparently.
One that 30 years on we haven't yet regretted 
I am sure you'll work it out your own way, you are bound to get lots of ideas here...
Just remember, it is supposed to be fun, reassuring, a good thing. If it isn't pack up and move on!