We aim to "Walk the Line" in our postures. We look for the correct alignment, the right geometry. When we find the geometry of a pose, the pose sings! Here are some props to help find the lines of Parighasana.
Take your bent leg to the wall - this lines up your back leg with the wall as well as your hip. Keep the hip against the wall. You can use a blanket under your knee or fold the edge of your sticky mat, if the floor is too hard for your knees.
Line up your straight leg with the bent knee. You can use the handy sticky mat edge to help you draw a straight line. The pose has an extended foot. I have a difficult time extending my foot, so I use a block to press into. If you can plantar-flex your foot well, by all means do so.
Loop a belt around your bent knee. Longer belts work better here, but all belts work. Line up this belt with your bent knee hip, side chest, shoulder and arm.
With all this lining up, you'll find it "very easy to be true" to the essence of the pose.
Ok, you say, that looks all lined up, and maybe there was even a little melody going on, but the pose in the book looks different:
Well, what about making the foot close to your hand? Place a block under the foot.
Ok, that might have helped. What about two blocks? Note: if you are using wood blocks, use a sticky mat in-between them so they don't slide.
By moving the foot closer to the arms, the hip comes in and it is easier to side bend. Once you have the side bend, go back to the non-propped version and see how far you can go.
Remember to keep your eyes wide open all the time.