Rocket League

From the Main Menu, go to Play Online > Find Match > and then in that menu press to switch to the Competitive playlists. Simply play ten matches in the same playlist and you will "place," earning you this achievement.

This Season is slightly different than before, so this achievement might pop under different circumstances. Before, the ranks reset and you had to re-place every Season. This time, ranks carry over. I can confirm that if you had Placement matches in progress (mine was 8/10 matches played), the achievement still unlocks when you place.

That being said, if anyone could confirm whether or not you get the achievement right away if you're already placed in every playlist, that would be great. Worst case scenario: If you are placed in every playlist and it turns out you have to do placement matches for the achievement, simply not playing in that playlist for a while causes your Ranked progress to drop a bit. So it's possible if this is the case to just wait until your rank is dropped back into Unranked status.

EDITS:

According to Stig Assassin, if you have already placed in a playlist, completing one match in it should unlock the achievement.
According to MCFishHD, if you place and don't get the achievement, try playing one more match to completion.
According to metallicafan459, if it still isn't working...

I tested a theory and it worked, so here i am. I literally just used this to unlock the achievement. As someone late to all the Rocket League "fun," I'm surprised no one came up with this yet. As we all know, comments in other guides don't count as solutions.

First, join the 1v1 Solo Duel competitive playlist. Once you load in and the countdown timer starts the game, simply forfeit the game. You'll earn items as well. After you forfeit and your opponent celebrates their good fortune, simply press "Find New Match" in the menu, and rinse and repeat.

Conservatively speaking, you should have this in 10mins. You'll notice that the game loading is the biggest delay for you. I'm curious if this method is even more efficient, time-wise, than the 1-goal, no bots, solo boost for items. I won't be testing it, but perhaps this will help someone else as well. Game smart, not hard.

So I've done this on two seperate accounts and I can confirm that if you had completed SOME but not ALL of the placement matches (I had 8/10), it will unlock after finishing the 2nd match and thus earning a rank. Also if you're already placed in a league then just play a game and it will unlock as soon as the game concludes.

From the main menu select Play and then select Play Online, Using and select Competitive.

From here you must complete 10 matches in one match type (3v3 Standard / 2v2 doubles / 1v1 Solo Duel / 3v3 Solo Standard) So for example completing 5 matches in 1v1 Solo Duels and then completing 5 matches in 3v3 standard will not count.

There are different criteria for unlocking this based prior gameplay, so I will list them separately below:

If you are already currently placed in a match type: You can play a single match in that match type and upon match completion the achievement will pop.

If you have completed part of placement matches in a game type (i.e.: 6/10 matches in 1v1 Solo Duel): You only need to complete the rest of the games to 10, so if you have 5 done, just do 5 more, and if you had 6 done, just do 4 more, etc. Upon finishing these matches the achievement will pop.

If you have not done any placement matches in a game type: You will need to pick a game type and complete 10 matches in that game type and the achievement will pop.