Make a list, Check it twice
Not just one, there should be a sanity check at every major step in the life of a layout. Some of us fondly remember the Apollo program on the grainy black-and-white TVs of the day. The whole build-up to launch was a long series of steps before the final countdown, Tee minus ten, nine, eight, seven... Liftoff. Looking at "T" as tape-out day and the steps leading up to it as a percentage, we can start at T minus 100 with 100% of the work left to be done.
T - 100
GATE 1 Design kick-off
Stack-up: It starts with defining the materials and geometries and capturing those as a baseline. Via structures, controlled line widths/gaps, minimum hole size, plating and finish specs are the first thing that drive cost and need to be known up front.
T - 99
Your fabricator(s) must be engaged at this point so that they have visibility and material ready for the big day.
T - 95
Outline: Mounting holes, connector orientation, keep-out areas, fiducials, cut-outs, assembly sub-panel, shielding, heatsinks, every mechanical attribute should be covered.
T - 90
You should have enough of a picture here to discuss the timeline and get buy-in from the team. If you're lucky enough to have a Program Manager, they start earning their paycheck right here. They're a huge asset and you need to be tightly coupled.
https://www.orcad.com/resources/blog/notes-board-room-checklists?utm_source=Burkhert&utm_medium=blog&utm_campaign=checklist