Backcombing babe, tonnes of it
Start at the roots and basically tease a comb/hairbrush through sections of hair in a backcombing motion, you'll get more savvy at this with practice. I'd suggest a light misting of hairspray when you've achieved the look you like .. as opposed to spraying directly on; this can be too much and adds weight, thus flattening. Or hey, whatevs works for you personally, differing hair types et al ..
*Disclaimer lol .. this can be tough on hair, so be kind and wash & condition prior to combing/brushing the hair out of this style. Enjoy sweet pea, have fun!