In order to check all available wagers, you need to press a special inventory button at the top-right of your hangar view:
Then in an opened Inventory window choose the Wagers tab:
In order to activate a wager, hover your cursor over it and press the Activate button under it.
Alternatively, you can choose a wager with a mouse click on it and then press the Activate button at the bottom-right of the window:
When you activate a wager you are prompted to make a stake (1), which affects the amount of awards (2) when you reach the steps of the wager:
Please, note that only one wager can be active at the same time, and if activate the second one, the previously activated wager vanishes along with your stake, and you don't get any rewards for it:
Your active wager is shown above the crew panel:
This is what neccessary in order to complete a wager:
I. You must comply with the conditions of a wager(1). If conditions aren't satisfied, the wager will be inactive, i.e. you won't be able neither to complete it, nor to fail it.
Pay attention: the Vehicles maximum rank in crew slots is a necessary AND the maximum allowed rank. That is, if it is specified that the maximum rank is III-IV, then it means that there has to be at least one vehicle of III or IV ranks and no vehicle of the V rank among your active vehicles.
II. You must achieve the wager conditions(2). If you complete a first stage of any wager, you will get to a second stage(3), etc.
However, if you fail, your counter of battles in which a new stage was not achieved increased by one(4). You have 3 attempts to complete a wager.
At an achievement of all available steps of a wager, it disappears, and you receive the award corresponding to your current wager stage.