Usually you can call your state agricultural department and either they have someone or can direct you to someone who can do it for you. As for price, everyone does it differently. A tester will order a bottle of the antigen ($100+ for a bottle with 1k test drops in it) and use it for all of their testing, so usually they try to make up that amount, plus they usually have to come to you unless it's only a few birds, so they will sometimes charge travel. The way ours charged was $1 per bird- that covered their travel, the antigen, the needles (and other supplies). Others will charge a fee based on a range of bird numbers (say 1-25 is $25 or something like that). I'm certified to do it, and I think I would also probably go $1 a bird. So I'm afraid pricing is very variable.
As for how it is done, you will have to make sure all your birds are where you can catch them and separate out the ones you've tested after testing so you make sure you get every bird tested (we did this by locking everyone up when they roosted for the night, and taking them out one by one, but our flock is fairly large... with just a few you may know them all by sight). The tester will take each bird in a brightly lit area, and make a pinprick on the vein under the wing. They will use a loop tool (usually the tool is a needle/loop combo like so:
http://www.cutlersupply.com/cart/index.php?main_page=product_info&cPath=3_81&products_id=458 ) and scoop a drop of blood. They will have placed drops of antigen on a testing plate of some sort (usually a plexiglass or plastic square plate with 1 inch squares carved into it, a drop on each square), and they will rub the blood into the antigen and let it sit.
A positive test will almost immediately look like someone put sand into the antigen. It's VERY obvious, but it's also very rare. If the test is negative, the antigen will remain looking like a purple-y blue liquid. If the test does come back positive, do not panic. The tester will usually re-test that bird after a bit, from the other wing. Sometimes it shoes negative after that, and the bird will be considered clean. If a second test is positive, they will bring the matter to the state, who will take the bird for further testing (the bird will probably not return from this unfortunately)
As the testers test, they should be recording all the information about every bird they test- gender and breed. So be ready to answer those questions. They are not required to band your birds, sometimes they do. They may or may not charge for bands, it is tester preference there.
At the end, you will be given one of the copies of the test results and probably have to pay a fee for the registration of your flock with the state (or federally, which you may as well do). The fee here is $25, but I don't honestly know if that is the same around the states. They will send in the paperwork, and once it's been processed, you will receive your certification and flock identification number. Every year you will have to be recertified. By getting the certification you are agreeing not to introduce non-certified fowl into your flock (meaning you only acquire eggs or birds who are similarly clean), and you also agree you are not exposing your clean birds to birds who are not clean. Doing either of those can void your own certification (and you paid for it, so it would be silly to void that!). You can, of course, acquire new birds and have them individually certified before introducing them to your flock if they have been acquired from questionable sources.
Hope that answers your questions!