It really is pretty simple. I had contacted this lab Horse Testing - Equine Genetic Testing
About testing a prospective buy, since I wanted to know the status for not only HYPP, but HERDA and other genetic disorders that may not be apparent just looking at a youngster.
On the site there is a form the Equine Testing submission form. Open it, print it, follow the directions, send the $$ and the hair samples as directed, and your results will be available usually within 2-4 business days of receipt of the sample. Accessible on their site (to the owner only) or via email. They will also send you a hard copy via snail mail. On the form is also the pricing. HYPP and HERDA are $30 each or $50 for both.
I have not used them, as we decided against the horse, but I did save the info and will use it if I look at another. Sure is cheaper than having the vet do it, which I had to do with a grade horse I had years ago who showed signs, but was negative. (his issue,believe it or not-the cribbing color sitting off circulation to his brain when he stretched his neck-causing seizures)
Good luck-IMO-it is worth the $$ to have the piece of mind.