Stupid question, but how do you guys learn new characters once you've ranked up enough?

I just hit Plat 2 (feel like I probably still belong in Gold, but who knows), but I've been pretty reliant on my main to carry me there. I have a couple other characters I'm comfortable but not competitive with, and I'm not sure how to go about training with them more.

There's such a huge skill gap between QP and Plat comp (not to mention the QP bots), that I feel like no matter what mode I choose with my secondaries or any new heroes I want to learn, I'm choosing between either steamrolling in QP or ruining the match for my team in comp.

How do you all become more or less equally proficient with 3+ characters?