Rihanna looked adorable while out in Miami, wearing her hair in a carefree mass of curls. All she did to create the look was draw her curly locks into a ponytail on top of her head. It's really that simple.
However, if you're lacking RiRi's natural spiraling curls, you can add a little texture using a combination of tools and products. One option is to set straighter hair on hot rollers, or try curling small sections with two different sized curling irons. Spritzing on a product like Frederic Fekkai Luscious Curls Wave Creating Spray and using a diffuser to blow-dry will also help develop more texture and body.
See Rihanna's Hair Lookbook: