Obliques are the muscles situated at the side of the torso. If while standing upright, you were to bend your torso to one side, that movement is done with the obliques.
Training the obliques tends to be recommended for strength athletes for practical application and performance rather than for physique and aesthetic appearance. Those naturally wide in the waist should avoid building the obliques as this detracts from the V-taper too much. Therefore, if you train solely for aesthetics, then there really is very little to be gained by trying to develop the obliques.
Having said that, if you wish to do something for them, I suggest not performing training on them using added weight, and taking the more ‘aerobic’ approach.
Anyway, if you wish to do oblique crunches – also known as side crunches – here is how you do them: lay with your upper back flat on the floor, and your legs together at the knee, tucked in towards the butt, laid out to one side of you. On the side your legs point, place your arm flat on the floor for stability. Now, with your other arm behind your head, lift your shoulder blades and upper back off the floor, and ‘crunch’ across into the oblique on the same side your legs are pointing, and slowly lower to the floor, and continue repeating for however many reps you wish to do.
Switch sides to work both sides of the obliques.