It depends on what kind of security your friend would want. Theoretically he can just give you the money and you pay it back when you sell. There's a bunch of other ways you could structure it but the most "correct" way to do it would be to record a mortgage and whatever other documents you'd need depending on your state. You'd need to pay for an attorney to do that and then there are recording and title fees associated that will cost a bit more.
If it's a standard fix and flip project, I think fair terms for friends would be 12 month term, interest only payments due monthly. The construction funds are typically released in draws where you would advance the work first, then he would inspect and reimburse you based on your pre determined budget.
If you funded it through a mortgage, your friend would basically give the $100,000 to the settlement agent. Any other funds needed from you for the closing would be given to the settlement agent as well. Then once closing is completed, they disburse the money to where it needs to go and they record the mortgage.