Do you know how to convert decimal to the likes of binary, octal or hexadecimal? If you do then exactly the same method applies, although i will assume you dont which is why you are asking this question.
I suggest you have a read of these and come back to us if you are still at a loss for how to do it:
I'll give you a hint, how do you multiply in decimal by long hand?
Think carefully as to what you are doing and apply it to your problem, EXACTLY the same rules apply, you will see that from the links i have sent your, the rules of maths still apply to any counting system.
This is an example, of a base 5 multiplication,
344 * 2 1243
Try and figure out whats going on here whithout me telling you.