• The top prize has been growing since mid-October, when two tickets split a $502 million jackpot.
• The chance of hitting the top prize is about one in 302.6 million.
• If you beat those odds, there are some things you should do — and not do — right off the bat.

Winning $640 million wouldn't be a bad way to close out the year.

That's the Mega Millions jackpot amount for the next drawing, which is set for Friday night. The top prize has been growing since Oct. 14 when it was reset to $20 million after two tickets — sold in Florida and California — matched all six numbers to split a $502 million jackpot.

While the chance of a single ticket hitting the jackpot is tiny — about one in 302.6 million — it's worth considering how you'd handle such a huge windfall. 

More from Personal Finance:
Consumers getting payments from Equifax data breach settlement
These 5 tips can help you knock down credit card debt faster
Here's how to maximize tax deductions for charitable donations

"The stress level is pretty high," said Susan Bradley, a certified financial planner and founder of the Sudden Money Institute in Palm Beach Gardens, Florida. "And that can be pretty rough.

"Most lottery winners are sleep-deprived for a while."

In other words, it's not as simple as claiming your prize and going on with your life.

If you happen to find yourself holding the winning ticket, here's what to do.

1. Breathe

There's no need to rush over to lottery headquarters right away.

Depending on the state where you bought your ticket, you get anywhere from 60 or 90 days to a year to claim your prize.

You'll also need some time to figure out how to handle the huge influx of money, according to CFP Janet Stanzak, founder and partner of Financial Empowerment in Burnsville, Minnesota.

2. Keep your ticket safe

It's important to protect your ticket. Experts recommend taking a photo of yourself with the valuable slip of paper and then storing it in a safe place, such as a lockbox.

"Protect your ticket," Bradley said. "People can lose tickets, and no ticket means no money."

It's also generally recommended that you sign the back of the ticket as proof that it belongs to you. However, before you do so, it's worth making sure you know the rules for claiming your win in the state where the ticket was purchased.

Some states allow you to remain completely anonymous. But if you bought the ticket in a state that requires the winner's name to be publicly shared, you may be able to avoid identifying yourself by claiming the prize in the name of a trust or other legal entity.

3. Tell as few people as possible

While you may want to share your good fortune, experts recommend keeping the information close. This means telling, perhaps, only immediate family or a trusted friend.

"You want to only tell someone who will hold your confidence and help you watch your back," Bradley said.

News has a way of traveling, and long-lost friends or family — or scammers hoping to get in on the windfall — could appear on your doorstep.

4. Build a team of advisors

Winning such a large amount of money means having some pretty weighty financial decisions to make. This means it's worthwhile having a team of professionals who can guide you. That group should include an experienced attorney, financial advisor, tax advisor and insurance expert.

Among the first big decisions you'll face is whether to take the jackpot as a reduced lump sum or as an annuity paid in yearly installments over three decades. Either way, the IRS — and the state, if it taxes lottery wins — will take a slice before the windfall reaches you.

For this $640 million Mega Millions jackpot, the cash option — which most jackpot winners choose — is $328.3 million.

A required federal withholding of 24% would shave about $78.8 million off that amount, reducing it to $249.5 million. However, the top marginal rate at the federal level is 37%. In 2023 — when the winner presumably would claim the prize — that tax rate applies to income above $578,125 for single filers and $693,750 for married couples, which means you could expect to owe more to the IRS.

Nevertheless, the amount you'd end up with is more than most people see in a lifetime.

"Ask yourself what this means to you and the people you care about," Bradley said.

Meanwhile, Powerball's jackpot is $246 million ($128.5 million cash option) for its next drawing, which is set for Saturday night. The chance of a ticket winning the top prize is about one in 292 million.