Do you have ready made loan shtar.
Unfortunately we don’t have any specific text for a shtar, there isn’t that much that you have to know in order to write it. You write for example, “I Levi Abraham borrowed $1000 from Shimon Israel on Tuesday July 1, 2020. The due date for the loan is December 31, 2020”. Then Levi signs on the shtar. If there are co-signers, they should sign on the bottom of the shtar, that they are guaranteeing the loan.
If the loan involves ribbis, then you would need to sign a heter iska, which you can find over here https://dinonline.org/2010/02/24/heter-iska/