This is quite likely the most important and enjoyable, phase of planning. Now, before you are buried under the details of running a campaign, take the time to scrutinize the road you intend to follow. Assumption is the mother of all great foul-ups. Place your starting forces and double check that each unit really is in the correct place with the correct orders. Nothing screws up a campaign worse than a dumb mistake at the start.
Planning is a learned skill.
The more of it you do, the better you get. You may never be a tactical genius but a solid, well thought out plan will keep you from being this games quick lunch. And you will get better each time you do it. Experienced players may not look like they're doing all of this, don't be fooled. Their experience allows them to operate with less formal planning but they have paid their dues and done it in the past.