Diffuse upside down. It takes a long time to diffuse my hair so when it is 60% dry I'll diffuse, focusing on the roots, and either upside down or with my hair tilted to a side I try to put the diffuser underneath my hair and move it upwards towards the crown to create more volume. Be careful not to disrupt your curl, if necessary do it by layers but always in a motion down to up, and then once you are done with the roots, diffuse the ends.In my personal experience is best to do it, with hair 60% dry and for each section start with high heat, go to low and end with cool. (it takes 2 min on each section) but that depends on how dry is your hair and the dryer settings. You can try a mousse that gives volume too, or try clipping your roots and removing the clips once dry.
how can i make my curls look more volumized? my curls are a little flat, i want my hair to look big
When you get out of the shower put your leave in with your hair upside down then I suggest you plop you hair with a microfiber towel or a tshirt. Plop for 20-30minutes. Put the rest of your stylers in if you have more (cream or gel). Then if you diffuse, diffuse with your hair upside down as well. If you air dry, don't touch your hair until it's 100% dry then put your hair upside down and shake your roots a bit to create volume.