How do I set up an adjusting period

Trying to set up Adjusting Periods after the fact seems dicey. I probably wouldn't recommend it. If you do, I'd test heavily in Sandbox first to make sure it doesn't hose your financials. You should just be able to add a new period and check the Period is Adjustment box. If you're needing to see financials with and without audit adjustments or something like that, I'd recommend instead just adding a checkbox to the JEs that says "audit adjustment" or something like that and then doing versions of your reports that exclude or include those entries based on use case.

setup your period of 1 day i.e. 12/31/24 to 12/31/24. call it Adjusting, make your journals into that period