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Could I get some assistance on where I should start on CP504?
Your client has an unpaid tax liability. Before sending the CP504B, The IRS notified your client of the amount owed. The taxpayer then neglected or refused to pay the tax. It is possible your client never received the notice.
This is a straight-up collect case. The best place to start is researching the issue to confirm the tax is correct and owing. If the tax was created by an SFR or some sort of adjustment, you should consider looking into correcting the info. If the tax is legitimate, you will need to work with your client to come up with a strategy. The IRS is concerned with one's ability to pay, so you should collect the financial info and based upon their standing figure out the best outcome.