Here's a super easy way to mix backing vocals but before you record them there's a few things worth thinking about.
Backing vocals really pop with multiple voices so if you have singing members in your band it's really beneficial to get them in and record them.
I know so many people work alone these days so if you have one voice for a project, using a different mic to the one you recorded your lead vocal with will help create difference between the lead and backing.
Once you've tracked them, send them to a separate bus so you can control them with just one fader.
When it comes to mixing backing vocals spend your time on your lead vocal. Around 95% of your mixing time on your vocal compared to 5% on your backing.
Once you're happy with your lead vocal, with this method, your backing vocals will just fall into place.
So now you can grab all of the plugins you have on your lead vocal chain and drop them onto your backing vocal bus.
Then make opposite moves to any EQ boosts you've created on your lead vocal. So +3dB on your lead vocal now becomes -3dB on your backing. You can leave any cuts you've made to muddy frequencies and your filtering too.
Next adjust the compression. Anywhere between 5 - 10dB of gain reduction, you can be more liberal than with your lead. Faster attack times will smooth out your backing vocals but too fast can sometimes make the bv's a little too thick and heavy.
Finally dial in some more reverb to send them further back in the mix and there you go.
Fast and effective backing vocals.