It is important to be able to execute the Rollover exercise confidently prior to attempting this exercise. However, there are modifications that help prepare for the full Jack Knife which will need to be used if there are any neck or back problems.
To start - lie supine on a mat, arms by the side palms down pressing gently into the mat, legs together.
The movement - breathe in and bend the knees, bring the legs into the chest, breathe out and extend the legs toward the ceiling, keeping the back on the mat at this point. Breathe in and lower the legs to 45° away from the body then breathe out and start to roll over, bringing the feet and legs over the body. Once balanced on the shoulder blades and the knees are over your face (specifically your nose), extend the hips taking the feet and legs toward the ceiling under control. Squeeze the legs together to keep strong, hold this position whilst balancing your shoulder blades. Breathe in, on the out breath slowly and under control articulate through the spine, imaging your vertebra are like links in a bicycle chain, back down to the mat and begin the exercise again.
Modification - perform the Roll Over exercise and from the legs being parallel to the floor, lift them toward the ceiling, 45° only to start and then straight back down to parallel. Repeat a few times. This will help strengthen the hip extensors and prepare for the full Jack Knife.
Note - when in the full Roll Over position, if you find it difficult to breathe or turn the head from side to side you have probably rolled over too far onto the neck. Keep balanced on your shoulders. You will initially use your arms to help you get your legs over and up but as you strengthen the abdominals and hip extensor muscles this will reduce.