Place a band around both feet. Lie on your back with both knees in towards your chest so that the knees are bent to greater than 90° and are above or past the hips.
Holding the right knee in tight (using your hip flexors) lengthen the left leg out into the resistance band whilst keeping everything else still.
The right remains in the starting position, isometrically contracting the psoas (hip flexors).
SLOWLY return the left leg to the start.
Do not let the lower back overarch. If needed you can place your hands flat under the curve of the back and apply gentle pressure downwards.
Repeat with the opposite leg.
Complete 3-5 reps on each side for 2-3 sets. Increase reps gradually when you feel able to (aiming eventually for 3 sets of 15 reps).
To progress you can reach the leg out as far as possible (think hovering off the floor). If you need to reduce the intensity think about keeping the leg higher and maybe not reaching out as far.