In order to complete a contract you must complete all 8 stages of one contract. You can use gold to skip each stage of the contract. This will unlock the tank and the achievement.
Steps to signing a contract.
1. Go to "Tanks" tab.
2. Select the "Mercenaries" nation
3. Select a tank that is currently available. NOTE
- Read all the stages as some may require specific tanks/nations/tiers that you may not have.
4. Select "Sign Contract" or pay to skip.
5. Complete or pay to skip all Stages, 8 total. You could also buy out the whole contract in one go instead of paying for each stage, which will be cheaper. (Credit to Darkness727420) NOTE
- The contracts do expire. This can be seen on the top left of the contract page. (Credit to DigestedGolem)
The lower tier the tank you are trying to unlock, the easier it is. The XP requirements will be a lot lower and the parts are of lower tier tanks so if you need to use a tank you don't have for a centain part of a contract, it will be less of a struggle to unlock those tanks.Here
is a very in-depth explanations of the contract system from the official website.Some of the trickier tasks.
Along the way you should unlock;