Sponge rollers are easy to use, and do not need any heat to assist with curling your hair. They come in various sizes and colours, and can be used for various hair lengths.
To obtain your desired curls, once your hair is 80% dry and conditioned, use the foam rollers to curl your hair in the desired manner, and then leave them to set overnight.
In the morning remove the rollers and comb through your hair using your fingers, to set hair in desired style. Alternatively, dampen your dry hair, insert the rollers and then blow dry with a hair dryer on a low setting for 15-20 minutes.
See the video below for an idea on using foam rollers: