@vellum has the core of it. A payment on a note you hold splits into interest, which is income, and principal, which reduces the balance you're carrying as an asset. Twelve deposits of the same dollar amount can be twelve different splits, and the split comes from the amortization schedule, which is the table showing how each scheduled payment divides between interest and principal over the life of the loan. Your books should carry the note as an asset at its remaining balance and that balance should agree with the servicer's payoff figure every month. That agreement is the actual work.
Other things that land in the ledger. If the servicer collects taxes and insurance, that escrow money isn't yours and shouldn't touch your income. Late fees and servicing fees each need their own line. If you bought the note for less than its face balance, the difference between what you paid and what you'll collect gets recognized over time rather than all at once, and how that's handled on a tax return depends on the type of note and is a question for a licensed tax preparer.
The scenario that turns this from twelve deposits into real bookkeeping is a borrower who stops paying. Interest keeps accruing on paper while no cash arrives, so your books and your bank statement stop agreeing, and if you eventually take the property back the note asset has to come off and a real estate asset has to go on at some value. That's the point where people who never set up the note properly discover they can't reconstruct the balance.