My diffuser is my best friend especially when I'm too lazy to dry my hair straight! My hair is naturally semi-wavy and with a lil' product and my diffuser I can achieve that islandy-beach hair.
For natural, loose waves:
Towel dry hair by leaving it up in a turban type thing. The twisting of the towel will already give your wet hair some waves. Once you remove the towel just let your hair fall naturally and don't brush/comb it out.
Next, apply some heat protectant/serum (I use RedKen's Heat Glide) and if you hair is dry, lightly spritz on a leave-in conditioner. Then apply a light hair spray or spray gel to give your waves some hold as you diffuse them (I use Pureology's Spray Gel or TiGi Frisky Scrunching Gel).
If you like the way your hair naturally waves, then just take small sections of your hair and kinda feed it into the cup of your diffuser. Dry the sections a little bit at a time on low-medium heat and at a medium speed to prevent the sections from blowing everywhere. I like to tilt my head to the side and let my hair lay naturally in the diffuser for looser not some voluminous waves. Bring the diffuser closer to your scalp holding the dryer almost on top of your head if you want the waves to sit higher.
If you have straight hair that doesn't wave at all, a diffuser can still work for you. It takes a lil more work though because you would actually need to twist sections of your hair, kinda like making ringlets around your finger, and use more product to help keep the twists together as you diffuse them. I've done this before using TiGi Catwalk Curls Rock Curl Amplifier and Curl Booster. To create the twists/waves, I'd pump a lil Amplifier onto my finger tips and twist sections of my hair. This stuff is really sticky but it dries soft and flexible. Once all the twists are done, I dry each section as explained above setting with the Curl Booster to lock in the wave.
When you're all done, add some shine spray and voila You're done!
In words it's sounds really lengthy and troublesome, but a diffuser is a lot easier to use that its seems.
Hope that sorta helps!