In the world of planning, the last thing one knows in constructing a work is "what to put first". Do you agree? How?

may be yes, maybe not, because after the establishment of planning, there is the study of priorities with different tools

Rather I say, we must use a systematic approach to identify where to start with....

If we are planning an activity, it is good to deploy a process map which can help you where to start with. Like, when you are planning a computer program, Algorithms will tell you where to start with.

A high level process map like - SIPOC and then detailed process map with swimlane, should help to identify where to start with.

Utlizing any type of process map systematically with key project members and team involved should become the first step in any planning process. Then this can follow with set of activities in each step, using the CPM to identify priority areas, etc

I didn't experience that. I have exerienced task rearrangement befor baselining the plan.

I think that this might be the case if you are working alone to plan something you didn't plan before.

Yes... everything begins with step1 or in common lingo, Priority. How? there's is Priority Matrix and MOSCOW to begin with...

if you are a dopting a strategic planning apporoch, thing will be different and easier, as you have many tools that allows you to know from where do you start. strategic planning as you know can be on all levels and not depending only on the top management level, you can apply it on departmental, or even a business unit level (terminology of business unit depends from corporate to another one, as i meant here a business unoit inside a department).