I read the requirement as being between you and the reflection in the mirror - internal record keeping. So you decide internally in your head or on a piece of paper with date or an email you send yourself. It’s supposed to be by Jan 1 of the trading year, or before the trading year actually starts.
Now I don’t know how the IRS would police that, because you don’t have to file anything with them about making the election (or opting out of 988).
Extra reading. Again, ask your CPA or tax guy.
At minimum, you’re filing 988, so the worst situation would tax your gains as ordinary income.