Basic Principles is you create 2 dynamics - the 'stake' and the 'risk'. So each card adds to, or reduces the stake and adds to, or reduces the risk of war (or of losing). Your ideal end state is a high stake and you win (the stake then converts into prestige) but sometimes you might want to manipulate a crisis to get the other side to declare war.
Its a bit more nuanced than that, but thats the essence of the system.
So at the start decide what you want/don't care about. Lets say you are Sardinia-Piedmont and its Prussia on the other side. Odds on, you really don't want a war. So the best cards are those that keep the risk low and the stake low. Try to manage it so you lose but its not a big hit on your prestige.
If you are Russia and its Austria, you might take the view that a war is ok. So go in hard, push up the pot and hope they back down (=prestige for you), if they don't well the war is an acceptable outcome.
A general rule is to play the agressive cards early, get the tension up and then play some cards to calm things down as your last pair. This should give you the pot, scare your opponent and hopefully avoid it all going out of control (unless you want it to be out of control).
When setting up your crisis cards you have 2 options. one is to choose a 'strategy' using a preset group of cards (on the right hand side of the screen), the other is to choose your own hand to fit your own strategy.