I'm sure it's been addressed, but speak to me like I'm seeing a bike for the first time.

Relatively new bike+apex group+suspected cable stretch.

Basically the rear derailleur double shifts when I upshift (to a smaller cog). Looking at my rear dr from the back, which way to I turn the barrel adjuster, and how far? Also, my front dr rubs when I'm on the small ring, is there a barrel adjuster for that?

Thanks for the help - I'm getting tired of going to the LBS for minute adjustments that I should be capable of myself.

Assuming it's just cable stretch.

You turn the barrel adjuster counter clockwise, looking down the axis of it to 'increase cable tension'. Do quarter turn of a time till symptom goes away. If that simple adjustment doesn't work you'll need to redo the entire rear derailleur adjustment.

It's the same for the front derailleur. Also do you know how trim works?