Well, there are several things that you need to keep in mind when building a winning tournament list.
#1. Be able to achieve the mission objectives
The first thing you need to do is pretty obvious: You need win your games.
This means not only to be able to crush your opponent, but take and hold objectives. You need to figure out on your own how to crush the armies in front of you, but for objectives you also have to pack a reasonable amount of troops. The other thing is that you have to get to where you need to go, so that means plenty of mobility. You either need to do it by having fast moving units, or a lot of transports, or durable transports.
In a lot of the larger tournaments they will have primary objectives, secondary objective and tertiary objectives so not only will your army have to be flexible enough to handle a lot of mission objectives, but you need to be able to do several at the same time.
Since TOs will generally be making up the missions, you will play in some missions that resemble the ones out of the rule book, but you will also get almost anything else that they can throw out you. You need to craft an army that can adapt…or you can just table your opponent.
#2. Conquer the unconquerable
The second thing you have to do is the hardest one and the most important: You need to beat the top armies. If you can't figure out how your list can win in a game against Mech Guard, Razorspam Space Wolves, and Battlewagon Orks you need to go back to the drawing board.
In a tournament you can get matched up against almost anything in the first couple of rounds, and if you have a good take-all-comers list you should be able to win if you are a better player than your opponent. After the first few rounds though you will start to go up against the heavy hitters and you will need to figure out a way to take them out.
So how do you beat the top tier armies?
That is a good question and it will be the subject of a latter post.