I know what you mean, there are countless of noob tutorials on it with mediocre and awkward sounds.
Growl are pretty much done in 3 different ways:
- Using FM, where you control a certain modulator's (or several ones') amplitude to achieve a growl (the best one imo).
- Using formant filtering, where you create a rough and heavy timbre, and let it run through some filters that in the end mimic the sound of a mouth (can create surprisingly good results sometimes, but can also sound completely awkward sometimes). This technique is the one people use the most when they do it in Massive.
- Using vocoding, where you again create a rough and heavy timbre, and let a recording of someone doing random sounds with the mouth be the modulator in a vocoder (not that experienced with this, but I've seen people who do it).
So dive into FM synthesis, and formant filtering, and vocoding.
Then, a lot of important stuff happens in the FX chain afterwards to take the growl sound to the level it has in many songs. Check out Resampling Synthesis, and perhaps also watch clips how people treat Reese-basses, since a lot of that processing is applicable to growls.
Lastly, check out SeamlessR on YouTube, if anybody knows what he's talking about when creating monsterbasses - it's him.