How to figure the payment on interest-only loans?

[Connie Brzowski]

Not sure if this is the right place to ask...

I'm trying to structure a business proposal for a private lender and I'm having trouble running the numbers.

This is from an article I read on another investment forum. Hope its okay to quote here (the author is Zack Wiest):

They lend you money at 10%, 12%, even 15% simple interest for a term of twelve months. You pay them interest only payments monthly, quarterly, or whatever you agree. You promise them you will not ask for more than 70% of the after repair value ensuring their loan will be protected by sufficient equity should the deal go bad.

So... using the trusty business calculator, how do I figure interest-only payments?

(if this is easy, I'm going to feel *very* silly 8) )