Step One appears easy, put off any decision until after the midterms. Trump is not a candidate, key issues driving the midterms (inflation, Ukraine, Roe) are not his issues and though Trump is actively stumping for many candidates, initiating any prosecution before the midterms is just too obvious. Nothing else about Mar-a-Lago has had an urgency to it (months passed from the initial voluntary turnover of documents and the forced search) and announcing an indictment now would be a terrible opening move. So if you're Garland, you have some time.

On the other hand waiting until after the midterms can be dangerous if as expected the Republicans do well and take both the House and the Senate. Even with slim majorities Republicans are expected to initiate their own hearings, into Hunter Biden's laptop and how the FBI played politics with that ahead of the 2020 election. Holding off an indictment until that is underway risks making your case look like retaliation for their case. That's a bad look for a Department of Justice which claims it is not playing politics. It would look even worse if the Republicans try and cut you off, opening some sort of hearings into the Mar-a-Lago search prior to an indictment. Nope, if you're Merrick Garland you are caught between a rock and a hard place.