1st track: in tune (let's call that "center.)
2nd layer: a few cents flat from center either by vso or just retuning.
3rd layer: a few cents sharp from center by the same amount via whatever method used above.
If you want it really thick, double each of the above parts, pan & mix to taste.
Yeah, to add to it.
Lead vocal, or guitar Center track. Create a send to 10 aux's to start with (just use a single send, you'll want to feed 100% of the signal to all anyway)
1. Pitch Down 3cents Hard Left
2. Pitch Up 3cents Hard right
3. Pitch Down 6cents Hard Right
4. Pitch Up 6cents Hard Left
5. Repeat ^ following same pattern but 9cents
6. Repeat ^
7. Repeat again but 12cents
8. Repeat
9. Octave down, low pass 400hz Centered, High pass wherever is needed
10. Morphoder Whisper track waves plugin Centered
11. Distorted Slapback Delay Start around 90ms Centered
You may want to create 4 more for your 1/8th 1/4 delays, and a room/plate verb. I work this way, I personally can't be doing with single main verbs that all of my tracks are being fed into, too much headache for me, my mac can handle creating them as and when I need them so....
If you have a stereo detuning plugin that can do the same detuning job as above then you can save yourself those 8 detuned tracks and instead use 1 stereo. eg: Waves Doubler
These types of things are done on typical pop records you hear. See it all the time. Some do similar with flex pitching and messing with formants but principal remains the same.
Technically the vocal has been layered 10-15 times by now, and that is if you are not adding parrallel compression etc, I do mine via insert on main track using mv2.
I'll note that tricks like these are done to guitar etc too. I asked Warren Huart this specific question and he told me that he did a lot of the thickening mentioned above to a lot of the Aerosmith records on those guitar solos.