Challenges are available from the main WRC menu (not from career mode).
Select them, and play each in turn to meet the objectives. When you've gotten gold on a challenge, you can move on to the next, although I recommend going for gold if you're comfortable with a particular challenge, and there's an achievement ties to that too.
I recommend doing the Training challenges, as they are the easiest, and have you driving as many laps as you need to get the target lap time at on of the test areas.
I also recommend skipping the Kenya extreme challenge - it'll have you tearing out your hair!
In order to unlock further challenges, you collect points by getting higher medals, so to skip certain ones you need to have gathered enough points to unlock the one after the one you wish to skip.
All in all, ten medals isn't difficult. Good luck!