The preparation should be done as early as possible.
It could start at pre-deal stage where the team has certain hypothesis about how integration should be done.
these hypothesis should then be tested and refined during the due diligence stage as the acquirer understands the target company better.
a draft integration plan should be prepared post due diligence, with the involvement of all relevant functional and senior management team
To institute the PMI plan can start as early as when the letter of intent is signed. At this point, there’s a level of commitment on both sides of the deal table. Starting to craft a PMI plan at this stage allows enough time to ensure that financial and strategic objectives align with the tactical implementation plans.
Right at the start of the process. Since post merger preparations often reveal specifics cross domains, that might otherwise be overlooked. This can then be an input for both a (changed) scope and/or (changed) negotiation perspective