What documentation is needed to deduct medical expenses?

You use your own records, receipts and bank or credit card statements.   Add up what you actually paid and were not reimbursed for.   And don't forget your health & dental insurance premiums.

Only what you actually paid during the year.  Medical is deductible the year when you pay it.  If paid by check, it would be when you gave the provider a check. You can only deduct the payments you made but not any interest.

If by credit card, it would be when the charge was made. Putting it on your credit card is the same as paying it.  You can deduct the full amount.  But you can't deduct any interest or penalties you paid on it.

If you put it on a credit card or took out a loan you can deduct the full amount even though you pay it over time. If you are on a payment plan with the hospital or doctor, etc. then it's only the amount you pay. Either way you only can deduct the principal amount, not any interest.

But You can only deduct the amount of unreimbursed Medical Expenses you actually paid over 7.5% of your AGI.  And then all your itemized deductions has to be more than the standard deduction to get any benefit (so you would only be getting the benefit of the amount that puts you over the standard deduction).

You don't send any proof to the IRS.  But keep your receipt, statements, and other proof for 3 years after filing in case of audit.