[header] How to do a jack knife sit up [title] Lie flat on the floor with your arms and legs extended. [step] Your arms should be lifted so that they're above your head. Make sure your spine is in a neutral position so your lower back isn't arched or rounded. 
[substeps] If you'll be exercising on a hard floor, lay down an exercise mat before you begin. [title] Take a deep breath before you begin.