This is a tough rank at first. You come in confident from the B ranks, since you were the top dog. Then you get dumped into a field where some players are actually S dropouts (maybe even S+ dropouts sometimes? lolol), and you're at the bottom again.
At this point most players have a pretty good understanding of a couple of weapons, what gear to use, and position-related stuff on maps. Also, they may have some good gimmicks that work incredibly well in B rank (but not A rank).
Personally, I just grinded it out. I took my losses as they came, and tried to adjust, over time, to the increase in speed and skill level. If I had to drop to B+ or lower than so be it, it just meant I needed more practice before I played with the As again. After a while I just got used to it, and now most A players seem like n00bs to me (especially A- players, oh god).
I also started searching for tips and high level gameplay on Youtube and Twitch, and asked a lot of questions about how to improve. This definitely helped a lot.