To unlock this achievement you need to first reach Level 3 in Unranked Team Adventure. From there, you can move on to Ranked Team Adventures. The rules and gameplay are identical to unranked games so you should be familiar with the mode by that point.
After reaching Rank 3, navigate to Pause Menu
> Team Adventure
> Ranked Team Adventure
. You simply need to complete your 10 placement matches to receive a league and unlock the achievement, even if you play terribly and get placed in a very low league you will have met the requirements.
Currently, the biggest struggle to obtaining this achievement is finding a match.