As of July 2020 there is no one playing online, so you'll need a boosting partner.
The menu is a bit convoluted, but you and your partner should go the the same lobby (can be anything, we used Asia 1\Tokyo 1.
Important: the rank is about the character, not the player, so if you switch characters you are essentially starting over.
One of you has to host (menu -> ranked match -> entry) and the other has to search and click X on the match, the host then gets a notification on the right bottom of the screen and has to press options and the X to accept the match. The host will still be up after every match, no need to recreate the "entry".
If trading wins, should take a little over one hour, about 20 wins and 20 losses, to reach level 11.
Note 1: If you are going for 100 ranked matches trophy as well, avoid switching characters too much, as it might be needed to be 100 matches on a single character. This is not confirmed, but many people around the internet did not unlock it at 100. I unlocked it at 101, 80 on one character, 1 with other and 20 with a third.
Note 2: if you are going for the World Tour trophy "Defeated a higher ranked opponent", you are essentially going to do the same solution here twice, plus some 15 more matches to get one person to Rank 14, on both instances. It seems the rank here works similarly to Tekken endless mode, you only get exp from opponents close to your rank, meaning when your boosting partner reaches rank 14 and you are rank 11, you'll get the World Tour trophy but won't be able to get any exp from him/her to reach rank 17 to reciprocate, hence you'll need both to level up other characters to 11/14 and do it again. After we did the first set, we played 30 matches straight and the ranked 11 didn't get to 12, so it is essentially a waste of time. So in total you'll need just about 110 ranked matches to do it all, give or take. And the 100 ranked matches trophy doesn't pop then, pick one of the characters you played the most and stick with it after that.