To work on your abs, constantly do exercises that force you to balance. The v-sit is one of such exercises. There are many ways you can do it depending on your body, your limbs, and how strong your core is. Start by sitting on the mat and using your hands to lift the knees towards the chest. Try to balance in the pose.
To go a level up, try to either stretch your arms in front of you or above your head.
If level 2 feels stable, then try to straighten your legs and lower your upper body just a bit back until you are really in a "V". Hold this for as long as you can. You should really feel it in your core.
Remember that it is okay to stop at level 1. Our limbs are different lengths and our bodies balance differently. It is better to be able to balance in level 1 than try to force a higher level that just isn't happening.
Here is collage showing the different levels.
Cheers Eights & Weights!