As the achievement description states, you need to earn 500,000 Black Market Credits to unlock this achievement. You can earn credits multiple ways, including:
* completing matches
* winning matches, including a boost for your first win of the day
* selling/discarding Burn Cards
* completing daily challenges
* earning XP after reaching level 50
This achievement is not retroactive and is cumulative from the moment you download Title Update #8. For example, my current status on the achievement tracker for this achievement equals my current credit count (about 29,000 credits as I write this), despite the fact that I've already earned several hundred thousand credits to buy the different Titan insignias from the Market over the past couple of months. Spending credits will not count against your cumulative total (thanks to TrueKill360 for confirming).
If you have additional information that should be included in this solution, please let me know in the comments.