Hua Gu San (also known as Flower Bone Three 55.04) is a point on the sole of the foot between the 3rd and 4th metatarsal bones. This point is located 2 cun posterior (toward the heel) from the web margin between the toes. If you can find Mu Liu 66.06 on the dorsal foot, this is the opposite side of the foot from this point.
Needling on the sole of the foot can be painful. I have a method I find to be far more effective than the "cough for me" and insert method I learned in acupuncture school. This method uses a long slow breath + a very strong exhale - like you're trying to blow out 50 birthday candles at once kind of strong exhale. Here's how it goes.
Drop your needle into your guide tube (assuming you use a guide tube - some people don't - I love them) and hang onto it with your non-dominant hand. With the fingertip of your dominant hand, massage Shang Liu firmly for 5-10 seconds. Massage in a circular motion for a bit, then press the tissue from the point outward 1/2 cun or so, visualizing making a space for the needle. You can even mentally ask the tissue to desensitize so that the point doesn't hurt when it is inserted. Yes, that's a little woowoo. So sue me!
After you've massaged for a bit, press the guide tube to the point and massage in tiny circles. While you are doing this, coach your patient through the long, slow breath exercise + very strong exhale exercise above. Demonstrate it, then do it along with them. Correct the breath if they aren't blowing out hard enough. You're still massaging with the tip of the guide tube while you're doing this.
Have them practice this one more time. Tell them you will insert on the next blow out breath and have them do the long slow breath one last time and on the strong exhale insert quickly and firmly.
This works well for any points on the sole of the foot.
By the way, "long, slow breath" is a more effective way to phrase this than "deep breath." Long, slow breath draws the breath very deep into the belly and is a calming breath which activates the parasympathetic nervous system.
If you say deep breath, most people will breathe into the upper body, which activates the sympathetic nervous system, causing more anxiousness about getting a spike in the bottom of the foot!