I've prattled on about LUFS for a few posts now: doling out a few meager tidbits of info and a smattering of real-world practicality, but now I want to talk about how to get you actually mix for Loudness Unit Full Scale. Well, at least drop the basics of how to do it.
I know I'm Jurassic School®, but I still think the best way to mix a track is make everything sound the best it can on a track-by track basis using whatever toys and talent you have in your arsenal while keeping the Master Output clean of processing. I occaisonally add a Brick-Wall Limiter on the Stereo Out to tame a peak or three, but using one of those Output Channel plug-in chains to keep everything in line is faking it IMO. Spending the time to make the mix the best you can is the difference between a passable engineer and a great one, and just plopping a preset in there won't make that happen.
There may be times when you need to add 'Mastering Effects' on the output for time reasons, but I still think that you can find the minutes to do a basic mixdown, take a quick break to clear the lugholes, and then listen back and make your final adjustments will give you a better mixdown than just digging through Channel Strip Settings to find one that works. That final mix is what starts the Mastering Process (we'll get to that soon) and you want to make it right, right? (right!)
For now, I want you to remember that your Mastering Engineer needs some 'wiggle room' (Headroom or lowered volume) in order to make your mix(es) shine. It's much easier to add than take away in The Mastering Kingdom, so maxing out the levels, overcompressing, and jacked up EQ's will only make the Engineer's job harder (and this costs you time and money dear Artist). So if you can get it right the minute it hits their ears and you'll get returned to you a track that will sound fantastic on everything you play it back on. You'll also make a friend who will be happy to work with you in the long run - not curse your name when your number pops up on the Caller ID. More on this later as well.
Alright already! How do I $#@! mix for this LUFS thingy?
Once that mix is tweaked and referenced on a few systems for translation (you are referencing your mixes on different sound systems, yes?), check the individual channel levels - is anything going over 0dB? You might assume you have a bit of level overage 'play' depending on your DAW, but I wouldn't count on it (I'm looking at you Logic...) Again, we're dealing with Digital here, and nothing goes above 0dB, ever. While we're here, it's also a great time to put a frequency analyzer in a Master FX slot to see if you're good on 'the curve' (Too much bass? Mids popped out? Screechy high end? All three?) Fix it now before you move ahead.
Yeah, yeah, use your ears and not your eyes we're all told, but a good Analyzer is a way to double check that what you're hearing is the truth.
Next take a look at your Master Fader or Stereo Output. If your average overall (RMS) level is between -10 and -6 you are in the zone. Anything higher and take that fader down until it fits in that range. Again, we're digital with 96dB of dynamic range for 16-bit and 144dB with 24-bit. You can reduce the range by quite a few bits without any change in sound or dynamics.
(Some will argue (and I would agree) that's it's better to reduce the level of the Channel Faders and/or processing and leave the Master Output at 0dB. It's a better way to mix, akin to using a traditional console, and is how I personally prefer to mix in the box. But you can take the Master Fader down 3 to 6dB in digital systems and leave the Channels alone with no noticable sonic damage if you're pressed for time.)
With that done, load a LUFS meter on one of the bottom slots (post-Limiting if you're using it) of your Master Output and play the song from beginning to end (if you stop for any reason mid-playback, reset the LUFS meter and start from the beginning again - this is the only way to gurantee accuracy!). The LUFS meter will analyze the loudness of the overall track and give you it's reading. We're looking for the Integrated loudness here: Short-term, Long-term, PSR, PLR, or any of the other terms don't matter for the moment.
So what did you see?
If you are showing around -16 or less on the Integrated Scale then you're golden. Pat yourself on the back and move on to the next project. If you want to make a Broadcast version as well, bring the level(s) down until the LUFS Integrated is showing -23 (or -24 if it's headed to Europe). Printing both versions is a good habit to get into.
However, If you are showing above -16 Integrated then quite simply it's too loud. It might also be too compressed or EQ'd (overcompression and hyped EQ can affect Integrated levels). Go back through track by track and do some taming, and then repeat the process above until you get the magic number.
Not that difficult now, is it?
It's exactly what you've been doing mix-wise all these years, only now we have a (legitimate) target to aim for. You can still 'RMS mix' to the Master as you get everything structured, and then check the Integrated when you're ready to mixdown. Once you get used to mixing this way (it took me about a week of off and on testing - your mileage may vary), you'll begin to get the feel for it and it becomes second nature - like everything else you practice. If your DAW doesn't have a LU Meter then I've linked two freebies to try out, and there are many more out there if you engage a bit of Google-Fu:
MeldaProduction MLoudnessAnalyzer: https://www.meldaproduction.com/MLoudnessAnalyzer?kw=analysis
TBProAudio dpMeter II: http://www.tb-software.com/TBProAudio/dpmeter.html
Welcome to the future, and happy mixing. :)