Retroactive pay processing for multiyear?

SOLVED

Can anyone advise me how to add retroactive pay for multiple years. In all of my reading, there are two scenarios. One is you take the annual amount, figure out the tax paid. Then you figure out the difference with the new amount and subtract that from the fist amount. Secondly, I keep seeing something about the amount being under $5,000 and deducting 15%. Any suggestions? I have already entered it on another income line as retroactive. Thanks!