donations received after years end- count in prior year

I hear there is a method to count donations submitted in 2018 but not received until 2019 but credited to 2018. Can someone direct me to that on-line location please?

You can find instructions at https://www.lds.org/help/support/prior-year-donations.

The donation must have been physically handed to a bishopric member or postmarked on or before December 31. A member cannot hand a bishopric member a donation on January 1 and ask that it be applied to the prior year, even if they wrote the check in 2018. The only legal way for a donation to be "submitted in 2018 but not received until 2019" is for the member to have mailed it to the bishop on or before December 31, 2018, with a 2018 postmark.

Keep in mind we're talking tax law here, not church policy. The church (and it's leaders) lack authority to declare any donation as part of the prior year if it didn't meet the legal criteria.

Edit: I should add that the strict tax laws for getting a payment credited to the prior year does not affect one's status of as full tithe payer.