I agree with HairObsessed on almost everything except diffusing because it does wonders for me
but I put my hair in the diffuser push it close to scalp (but not all the way so that it doesn't touch my scalp) and turn it on for a moment, then turn off and repeat all over my head
best results for volume - doing this but with head upside down
best results for curl and root volume at the back - to add product and concentrate for a moment on the top layer of hair in the back - a second mirror comes in handy so that you see what you're doing
my hair is less frizzy after diffusing that without it, but the key is to put a lot more product (leave-in + extra hold gel) on - to saturate the hair - and remember about product in the back, oh and I don't SOTC after less than an hour from using the diffuser - it lets the hair dry completely so that SOTC doesn't disturb my curl when it's completely dry (it can frizz up if you SOTC slightly damp hair)
but I guess diffusing is sth that some people like while others don't - a personal preference