Keep in mind I am NOT an accountant or tax expert--but this is what I've done since 1986 when I started getting paid for my writing:
Do you pay taxes on your writing advances?
Yes. I fill out a Schedule C and pay the Self-employment tax, which you still have to fill out even if you have a day job. Writing has been considered "Miscellaneous Income" in the past.
Are office supplies (paper, ink, envelopes) considered a tax write-off?
Yes, if they are for your business. You will enter those costs on the Schedule C form--
SAVE YOUR RECEIPTS!!!!!! ALL OF THEM!!!!--and deduct them.
I deduct the percentage of space in my home that is my office, along with the percentage of utilities that apply, along with long distance charges, reference books & trade publications--anything specifically to do with my writing business.
Or do you just get to collect your advance, as well as your portion from store sales, for yourself after your agents' commision?
There's no free lunch, sorry. Uncle Sam wants his cut of your earnings.
Your publishers and/or agent will send you a "Miscellaneous Income" form in January, which will have the total of all money paid to you for that year. The amount will go on the appropriate IRS Schedule forms. The miscl. income form will include all your royalties. If the total amount is zero, you will enter zero on your return. You will include a copy of the form when you send in your return.
Note that book royalties are different from gas royalties, should you own a gas or oil well.
My advice? Put at least half your advance in a safe place and forget you have it until April 15th.
Or make quarterly payments to the IRS--I make them electronically direct from my bank to them four times a year. My estimates are always short, but it takes the edge off. If I don't have enough to pay my taxes, I arrange a payment schedule. So long as I pay X amount every month + interest, I have been okay with them.
Your publisher/agent doesn't make the automatic deductions you get from a regular "day job" employer. You will owe taxes on your earnings as a writer. You will pay self-employment taxes as a writer.
Many writers have accountants. I can't afford one and learned to do my own taxes.
I LOVE Turbo Tax. I got the one for small businesses and it hauls out deductions I didn't know about. Since I bought a house it figures out that extra complication too. Now instead of a week to do my taxes it only takes a couple hours.
BUT--- you HAVE to have your receipts for all your deductions. I total them up each month and keep them in a spread sheet file. The physical receipts go into a box for that year. When I'm done for the year I seal the box and put it in a safe place with all the other tax boxes.
The IRS doesn't care
how you make your money. Just give them their cut and they'll be happy.