Do you need to clearly write down your plan on a piece of paper or can you plan something mentally?

In Planning Process, documentation is your close friend. Always starts to write down your plan with the end in mind. You may start your plan with a mind map, but you will never see the big picture if you don't lay down your plan ( ideas ) on paper. 

Both are important , because without mental plan we can't proceed for paper plan, so both are important.