Quite often people who cannot get a loan from the usual sources (such as a bank) turn to family members or close friends to help them out. And you will probably feel a personal obligation to help out. If you offer to draw up a loan agreement or a promissory note, the borrower may feel offended that you don't trust them, that their word isn't good enough for you. This then adds a good measure of guilt to the feeling of obligation. Do not let your personal feelings sway your judgment. There's a reason why a bank won't lend them the money, and for that same reason you shouldn't lend them the money either - unless they agree to put it in writing.
Set the interest rate at a reasonable level. Check with your bank to see what its prime lending rate is, and add another percentage point. That's as good a rate as the borrower could expect to get on a line of credit. Make sure the regular payments are high enough to pay off the debt within a reasonable time period, but not so high as to be difficult to maintain. And get some collateral security, such as a mortgage, or a pledge of vehicles or other assets which can be sold if the borrower defaults.