Lay on your side with your elbow down and either your knees or feet stacked depending on the degree of difficulty you want. Feet are harder. Whichever you choose, your body needs to be in a straight line from your head to that point. Try to keep your spine as neutral as you can.
Slowly lower your hips to the floor then squeeze your obliques to raise your torso back up to neutral. You can also start the movement with a static hold for time followed by reps.
3-4 x 25 reps will usually get the job done. If that’s easy, then make it harder. Words to train by!!