Terravek has acquired two companies: Photonwerk in 2016 and Cardinal Grid in 2021. I led the operational integration of the second and inherited the lessons of the first.
The 2016 integration was done by instinct by people who had never done one. It worked, eventually, and it cost us about eighteen months of productivity and four people who left because nobody told them what was happening to their role.
After it, Saskia asked for a written account of what we had learned. Producing it was resisted by everyone including me, because it read as bureaucracy and because the honest version was unflattering.
That document is why 2021 went better. The specific things that came from it: name the integration lead on day one and give them authority; publish a role-by-role answer within thirty days even where the answer is 'not yet decided, decision by this date'; do not rebrand for at least a year; and identify the three things the acquired company does better than you and protect them explicitly.
That last one is the hardest, because an acquiring organisation's instinct is to assume its own practices are the reason it could afford the acquisition. Cardinal Grid's safety assurance process was better than ours and is now the group standard.