Do something standing on your head
Definition 1 of 1
Pro Tip 1/3Metaphorically Easy
The phrase is used metaphorically to describe a task so simple it's like defying gravity. 
I've practiced so much that I could do this dance routine standing on my head.
Pro Tip 2/3Not Literal
Remember, it's not meant to be taken literally; don't actually try to stand on your head! 
She knows the software so well, she could configure it standing on her head.
Pro Tip 3/3Informal Expression
The phrase is casual, so it's best used in informal situations or conversations. 
With all his years of experience, he could do the job standing on his head.