Modified Plank: box or countertop

Start with your elbows extended, legs extended, and bum squeezed while placing your hands on the surface of the box or countertop.
Make sure you actively PRESS away from the box/countertop so your shoulder blades are NOT pinched together. Think about spreading your shoulders apart.
At this beginning stage, it's ok if your upper back rounds a little bit as you push away. Try not to round, but actively thinking about pressing away from the box will be helpful.