Advice needed on how to enter into an agreement & hold title with a partner

[Kris Schwartz]

Seeking advice on how to formalize an agreement with a partner to flip a house together and how to hold title assuming that we would not be creating an LLC until after a "trial period". What documents would you require before you contributed funds in any amount?

Quick searches include terms like Joint Venture and Contractual Agreement. Is this something we draft ourselves including outlining roles and responsibilities, are there templates out there to use, or should we hire an attorney? What am I missing that needs to be considered? 

One additional detail, what if one of the partners has and existing LLC and the other does not?

Thank you for your insight!