Once you've achieved a 1st place win in a 4 star (silver wreath) championship for each of the five event types, the career mode will start to randomly dole out gold wreath world championships along with normal 1, 2 and 3 star events.
All you have to do is achieve a 1st place in each activity (again) and you'll have the tasty Mythical achievement.
If the type of gold event you wish to complete doesn't appear, or if you're in a hurry, simply enter and quit 1, 2 and 3 star events of the same type until the gold championship appears. For 2 and 3 star events use 'Quit Activity' from the pause menu after the race has started. The activities menu will normally offer you 2 star events after quitting a 1 star, and 3 star events after quitting a 2 star event (and hopefully a gold 4 star event after quitting a 3 star). This may take several attempts but I found a gold championship would appear after quitting six or seven race events in succession.
The difficulty level seems to be on par with the 4 star silver championships. If you're having trouble beating the ludicrous Rally AI times, there is a way to reduce your driver level that the AI uses to generate the stage times. Go to the recruitment menu and note the level of the recruits you're being offered. Mine were around Level 20. To lower this level and receive easier times to beat, enter and quit as many 1, 2 or 3 star events as required to lower your driver rep (and therefore the available recruits) down to Level 10 or 11. Using extreme-khana or buggy events works too. Quit each race (or quit activity if it's 2 or 3 star) and check that your potential recruits level is dropping. I got mine down to 11 and managed to win the Rally world championship (using the fully upgraded Cat 7 Citroen) without too much swearing.
One last thing, with the Rally Cross championship, don't do what I did and assumed that I beat the first event's qualification round - I clicked through at the end of the race without checking the overall position out of the 15 racers and didn't realise I was well below the posted 1st position overall time. You'll need to win all three events if you have a two or three event Rally Cross championship! Always check your overall position before you click through to enter the semi-final, and if you're not in the top 5 at the end of the qualifier, restart and practice until you are! I got 2nd place for the championship and had to enter/quit several more events for another gold championship to pop up.
Have fun! Poops out.