Long Term Liability needs to add interest to balance

Hello -

I created a long term liability. I basically borrowed $40k from my Mutual fund. I created an account for the loan. As I have paid it back, I write a check in QB for the total amount, and in the detail, one line is the amount of principal I am paying back, and in the other I get the interest from my mutual fund statement each month, in the section relating to my margin loan.

One month, I sent a check for less than that interest. So the interest that I didn't pay added to the amount/remaining balance of the loan. How can I have that reflected accurately in my loan balance? I have never done a journal entry - that intimdates me...

Thanks in advance,

chris