Today I filmed THIS video of a psoas awareness variant of the bolster hip flexor stretch, from the Stretch Therapy syllabus. As I mentioned in the clip, for me I can get right onto the deep hip flexor (psoas) using this position and sequentially contracting different bits of my leg.
The sequence of contractions I found worked for me was: shin into the bolster (knee extension); tail tucking then finally hip flexion with a straight-ish leg. You can also do stuff up in the spine and torso (and with the opposite leg), but it's a little bit hard to explain in writing. If you have high level body awareness you'll figure it out pretty easily.
Also, the methodology for the exercise is different than the standard contract-relax method used in a large number of Stretch Therapy™ stretches. In this, I use the contractions to get a sensory awareness of psoas contracting - and then I hold it. I might contract it, then over-contract it (modulate the tension from medium to high, then back to medium) - but I never full relax the muscle. You add other movements to increase the stretch on the deep hip flexor (lateral flexion; rotation; rotation and extension (if no back pain or dysfunction prevents this)).
There are other complex things (like adding in additional spiral contraction vectors) you can do to enhance this stretch, but as I said above, until I figure out a way to accurately describe what I'm doing, you'll just have to explore yourself.
A few people who tested this stretch reported just a standard hip flexor stretch, so I am musing about whether you need to have relaxed the rectus femoris muscle sufficiently before this stretch actually works for psoas..
My current physical cultivation explorations with stretching and stretching related body-mindfulness practice, have lead me to some interesting sequential contraction stretching techniques (which I will continue to post up for people to try). Hope it works for you! Go easy on the intensity, this is a 'introspective' stretch.