Begin with the part that will teach you whether the whole plan makes sense.

Most plans contain one uncertain piece and several comfortable ones, and the natural order is to do the comfortable ones first. That builds momentum and buys nothing, since the unknown is still sitting there able to invalidate everything already finished. Starting where the doubt lives feels worse and is the only step that changes what you know.

Identify the assumption your plan would fail without, and test it this week in the smallest way available. One phone call or one trial run early can save a month of well-organized effort.