If this isn't a case where you would be willing to forgive the debt if they can't pay, it's a business transaction, not a friend transaction. Establish exactly what the interest rate will be, what the term of the loan is, whether periodic payments are required, how much is covered by those payments vs. being due at the end of the term as a balloon payment, whether they can make additional payments to reduce the principal early... Get it all in writing and signed by all concerned before any money changes hands. Consider having a lawyer review the language before signing.
If the loan is large enough that it might incur gift taxes, then you may want to go the extra distance to make it a real, properly documented, intra-family loan. To do this you must charge (of at least pay taxes on) at least a certain minimal interest rate, and they have to make regular payments (or you can gift them the payments but you still won't up paying tax on the interest income). In this case you definitely want a lawyer to draw up the papers, I think. There are services on the web Antioch specialize in helping to set this up properly, and which offer services such as bookkeeping and monthly billing (aT extra cost) to make it less hassle for the lender.
If the loan will be structured as a mortgage on the borrower's house -- making the interest deductible for the borrower in the US -- there are additional forms that need to be filled. The services can help with that too, for appropriate fees. Again, this probably wants experts writing the agreement, to make sure it's properly written for where you and the borrower live.
Caveat: all the above is assuming USA. Rules may be very different elsewhere.
I've done a formal intractability mortgage -- mostly to avoid gift tax -- and it wasn't too awful a hassle. Your mileage will vary.
