After much trial and error, here is how I do it:
I don't use the diffuser anymore.
What I do is put product in my hair, let it air dry anywhere from 30 to 40 minutes then I use the attachement on the dryer that is for when you straighten your hair. On high speed, high heat I take cup sections of my hair and point the dryer at the roots and mostly the underside of my hair and I only do it for 3 to 5 minutes, though mostly it will take LESS than 5 minutes to do.
I never thought I would get good hair this way but I need to blow it dry for volume and curl definition. I HATE how it looks when I air dry - it's just too flat even though my hair is thick and coarse. The diffuser was taking too long so one day I didn't have the time to spare 10 freakin' minutes holding a diffuser in sections on low speed/low heat only to have it come out frizzy anyway because it was taking too long to dry and I was messing with it to much during the drying process with the diffuser.
This way, I mostly air dry, then blast it with the blow dryer and I am done in half the time with way better curls, since I am not messing with it as much. If I try to diffuse or blow dry right away I will ALWAYS, no matter what, get dry looking, frizzy, flat yucky hair. ALWAYS. And if I am stupid enough to blow dry right away I HAVE to blow dry it till it's bone dry or it REALLY looks horrible and that takes FOREVER and is not worth the effort! I came across letting it air dry first by accident and I never looked back.
Hope that helps!
HG's: Dep Sport Gel used over a leave in (I alternate between a moisturizing LI and a protein LI, depending on what my hair needs) and Giovanni LA Hold Hair Spritz
Method: Air dry for 20 to 30 minutes, then blow dry the under sections for about 5 minutes for volume.
2/3 mix waves/curls - coarse, frizz prone, med. thick, porous, at-home color for gray, protein sensitive but I still need it sometimes, and I can't use glycerin in high humidity, NOT CG, wash with regular S&C 2x a week.