I look for analogous situations, remember what I did in that situation, and then try to imagine what would happen if I applied it in this situation.
Like others, I break thinks down into smaller steps, and then figure out which order I should perform those steps in, given the availability of the resources needed at that time. Some steps may depend on the results of prior steps, so that helps arrange the plan in a sensible sequence.
So I employ imagination, memory, logic and analysis in order to create a plan.