To begin, lets go over the first steps of preparing for a mission.
1. THE ARMIES: An agreed upon points limit. This is obvious enough. I usually favour 2000 pts as it allows me to field the majority of units I like. In addition, this points amount is large enough to allow what little synergy exists in 40K as well as some redundancy if you decide to go down that path with your army list. Regardless of the point size of the game, it's imperative you include plentiful amounts of troop choices to secure objectives.
2. THE MISSION: Once again, simple enough. You either roll or just decide on a mission. Knowing which mission you're doing ahead of time obviously allows list tailoring, but I'm generally against that so an all-comers list is what this article will be based upon.
3. THE BATTLEFIELD: A roll-off or decision on what kind of deployment will be used. Dawn of War, Hammer and Anvil and Vanguard Strike all guarantee 24' between you and your opponent. This is NOT a lot of distance at all. It's crucial to realize that 24' can effectively be traversed by a lot of vehicles and more importantly, transports in two turns. Second turn assaults are certainly possible with turbo-boosting bikes and Fast Skimmers (Looking at you Dark Eldar), although unless the entire army revolves around such a blazing assault, such forward elements will be left unsupported.
If you're using Fortifications, they're set up before terrain. The larger pieces such as the Imperial Bastion or Fortress of Redemption should be done in a way to either, a) provide maximum height and opening up of firing lanes for static elements of your army, and/or b) provide a forward piece of terrain to attack incoming foes, whether they be ground or air-based, and/or c) if nothing else, provide a juicy distraction in the middle of the board and/or near a potential objective so that your mobile forces can attack and secure objectives. I could go into this more, but instead I'll reserve Fortifications and their strengths/weaknesses for a full article later!
The third component of The Battlefield section is placement of terrain. This is a highly subjective part of the chapter that is dependant on what terrain you have available, how much of it there is, what you will actually be using and their size and purpose on the battlefield. My general advice is being mindful of maximizing firing lanes and limiting or eliminating placement of terrain that can easily conceal approaching assault units.
The final part of The Battlefield section is the placement of objective markers. Once again, obvious enough. Advantage goes to the player who won the roll off to see which side of the table they want to deploy in if there's an odd amount of markers. Always deploy markers in a way that ensures you have clear line of site so you can bombard them with fire-power. Simultaneously, do your best to prevent the same for the enemy. Objectives that are difficult to see/approach by the enemy make life a lot easier for you.
4. DEPLOYMENT: Determining Warlord Traits is practically an article in its own right, so I'll keep it brief by remarking that your choice is highly dependant on the purpose of your HQ choice(s), although I generally think Strategic Traits are best since they can affect a large portion of your army or the enemy.
Deployment of forces commences with another roll to see who deploys and goes first, and who deploys second and goes second. First and foremost: WIN THIS ROLL OFF! I am a strong proponent of going first. Deploying first can force your opponent to avoid firing lanes/tar pits you've set up, gives you board control through first turn movement and as a result potentially enables the alpha strike component(s) of your army to decimate the foe with concentrated fire-power. Saying all of this should give you indication that I play aggressively and always want my opponent to react to my movements.
Conversely, going second means you must be mindful of the alpha strike potential of your enemy as well as your ability to deal with first turn incursions into the mid-field and your own deployment zone. Bids to capture objectives in your deployment zone should always hold priority if the opponent over extends himself, as it will be easy to cut off and surround his units. Once again, deep strike and infiltration should be observed as possible back up, for you or your foe.
To finish Part 1, I'll touch on understanding deployment of your forces and vehicles in particular. Going first guarantees mobility but will also put your units closer to the enemy. Do everything you can through naturalized cover, artificial cover from your own vehicles and upgrades/tools such as smoke launchers to maximize Cover saves. Mistakes always sting, but losing a transport because you didn't park your first Rhino correctly to ensure cover for the remaining ones can lose you the game. One particular article that everyone should read is by Stelek from Yesthetruthhurts.com. This covers Rhinos nicely in relation to deployment and first/second turn movement.
Beyond vehicles, placement of units in cover is primarily for static firebases (Longfangs), Gunboats (Predators/Leman Russ), and Dakkadreads while advancing hybrid (Grey Knights) and assault units (Guys with knives) should start in the open and advance to cover if going first, or start in cover and hopefully advance to more cover if going second.
That's it for now! Part 2 will detail the rest of the chapter and further detail Victory Conditions, Mission Special Rules and suggestions and advice for each of the main rulebook's six missions!