Journal entry confusion

I believe I understand Dr/Cr.

My confusion arises with a reversal journal.

If I had to make a reversing journal at year end, but then found an error in the next month upon review, how does one rectify?

Initially i thought to post a reversing journal in the next period, but this might not be necessary if the journal simply reversed back anyway?

I get very lost here, is there an easy way to get a better understanding of journal entries and reversal journal entries, especially if a mistake is made during the original posting of a reversal journal?

Thank you