You can take Singulair, since it blocks leukotrine only, not histamine. Hopefully they'll call you in a scrip. That period of time before the test can be awful, but it's much better with Singulair.
My allergist does the testing on my upper arms; I think some practices still use the back (more space to work with but a bit more hassle for the patient). This is how they've done it for me for years. I don't know if other places do it differently.
A tiny amount of each allergen is injected just below the upper skin layers, you'll wait a few minutes, and then they'll look whether a wheal was raised, and if so, measure the size of it.
They'll start with a single injection of histamine just to make sure that you're reacting.
Then, they'll inject you with each of the allergens you're being tested for. (Hopefully you completed a history so that they know what your potential triggers are, but if you have symptoms in every season, it may be a very long list.) They'll write on your skin to identify the allergen and the concentration.
It's been a long time since my very first test, and I don't remember specifically, but have to guess that they start with the weakest dilution of each. (On a re-test, they start with the dilution that worked on the last test.) After you've been injected with everything, if you react to something at that concentration, IF there's a weaker dilution they'll try you with that (but as I said, with your first test, there may not be). If you don't react, or if you react very weakly, they'll try the next strongest dilution. They'll keep going until you've either gotten to the strongest dilution without a reaction, or until they find the dilution that gives you a wheal of a particular size. (I believe it's 6 mm, but not sure.) They'll go past that one dilution just to be sure. The whole thing will take 2-3 hours.
So you'll wind up with a matrix of prick marks, itchy wheals, and pen writing. You'll have to avoid heavy sweating or washing the area for 24 hours so the writing doesn't come off (that's why doing it on your back is more hassle ). You won't be allowed to put anything on the wheals to relieve the itching. The next day, you go back to the allergist's office so they can check the wheals and make sure you didn't have a delayed reaction to anything.
Once the testing is done you can have your first shot. They'll start at a very weak dilution of everything, and build you up gradually until you've reached your therapeutic dose of each allergen. After each shot, especially when they're building your dose, you'll have to wait around the office for about 20 minutes to make sure you don't have a severe reaction. My allergist's office does shots twice a week while I'm building my dose up (at a full dosage, it's every 5-7 days). Yours may limit you to once a week. How long it takes to build you up varies, but it's usually about 6 to 10 weeks.
Once you're on a stable dose, your allergist MAY let you give yourself your own shots. Not all of them do...but it makes the process WAY more convenient when you don't have to go to the office every week.
HTH. Good luck!