Wet your hair, either in the shower or with a spray bottle. Your hair should be damp, but not dripping down your back. If you just stepped out of the shower, squeeze your locks with a towel and give your hair 10 minutes or so to dry a little.
Comb your hair with a wide-tooth comb to get any tangles out of it. Use the comb to part your hair the way you want it. Squirt a good amount of mousse or gel into your hand and run it through your tresses.
Press your index finger down on your front section of hair, about 1 inch from the part. Take a fine-tooth comb and place it next to your finger. Drag the comb back against your finger, pulling all the hair below it back. This creates a wave. Flatten the comb against the section of hair and place your index finger and middle finger around the newly created wave.
Comb the hair below your finger down so it creates a curve in the opposite direction. This creates a ridge of hair in one direction and a C-shaped wave of hair going the other way. This is your first finger wave.
Move your fingers down about two finger widths and press your index finger down onto your hair. Repeat the finger wave in the same way you made the first one. Keep doing this down the section of hair, and move around your head making more waves.
Let your hair air dry to lock in those soft retro waves. Brush them out for soft natural waves, or just spritz them with hairspray to create a real flapper look.