The directions come with the vaccine, I found them hard to read because of the small print, but easy to follow. When you buy the vaccine, make sure you also get a good supply of hyperdermic needles. According to my vet, you should use a new needle for each chick.
It is expensive to ship, because it needs to be kept very cold, so you end up paying for freezer packs and a styrofoam box which costs more than the vaccine.
I took pictures when I did it this spring. I'll try to remember to post them.
Even with the vaccine, I lost most of my chickens to marek's this year. Apparently there are different types and my chickens must have gotten a different type than the one the vaccine is for. My neighbor figures it is like getting the flu shot, it only works if you are exposed to the right kind of "bug".
By the way, not all of my chickens are vaccinated, and amazingly none of my "homegrowns" (of which only some were vaccinated) have come down with marek's.