The pH test is, of course, simple with your pH meter. These meters come in units the size of a pen to bench models for the lab. I recommend getting the best one you can afford. It will be used for testing acid levels also. I also recommend getting one with an automatic temperature correction. Otherwise, you will need to make sure your samples are always the temperature recommended in the instructions that come with your meter.
You generally want pH levels between 3.2 and 3.6. If the pH is over 3.6, it would probably be a good idea to lower it a little. High pH wines are prone to instability and very susceptible to spoilage organisms. In addition, the higher the pH, the more sulfite one needs to add and maintain as an anti-oxidant during the winemaking process. If you go to http://winemakermag.com/sulfitecalculator, you can bring up a calculator that will let you fill in the pH and tells you (at the bottom) how much sulfite is needed for that pH. The only problem is that they want to know in liters how much wine you are treating, so you have to multiply your gallons times 3.785. It is a very simple calculator but fits the purpose. You will see very quickly that you need quite a large amount at, say, pH 3.8. It says 80 ppm.
The relationship between pH and sulfite addition is very important because the higher the pH, the more sulfite you need. The legal limit for sulfite in commercial wine is 300 ppm, and there is good reason for this. Having a lot of sulfite in wine can cause various problems. It strips color, can cause bitterness, and have a very adverse affect on people who are asthmatic or just allergic to sulfites.
When you add sulfite to a wine, a fair amount of it gets bound and some stays free. The only sulfite that protects the wine is the free SO2. If it needs 80 ppm, that means 80 ppm free. There are factors too numerous and technical to share here relating to why sulfite gets bound. The rule of thumb is that at least ½ of the sulfite you put in gets bound. Usually more gets bound when wine is very young and as it moves along its development in tank, barrel or carboy, more of it stays free. So you should expect that, if you add 50 ppm sulfite, you may get 25 ppm free. You can see that you could get to the legal limit way too quickly if your wine is 3.8 pH.
In order to avoid this problem with high sulfite addition, you can lower your pH with tartaric acid. 1 gram of tartaric acid per liter (or 3.786 grams per gallon) will lower the pH by .24. 1.3 grams tartaric acid is equal to ¼ teaspoon and 5.2 grams is equal to 1 teaspoon. So, if you have a juice that is 3.8 pH and you want to get it below 3.6, your calculation for 5 gallons would be: 3.786 g X 5 gallons = 18.93 g = 3.64 teaspoons or just over 3 and ½ teaspoons for the entire 5 gallons. Always mix the acid in a little water until it dissolves before adding it to your juice.
If, for some reason, your pH is still too high after fermentation (when you should check it again), it is possible to lower your pH without increasing your acidity, or increasing it only very slightly, by adding tartaric acid just before cold stabilizing. The tartaric acid addition lowers the pH as above and then precipitates out during cold stabilization but leaving the pH at its lowered level. If your pH is low, say 2.9 to 3.1, or lower, you have probably picked your grapes too soon and they will not be ripe. At that point, there is not much you can do about it. Consult my earlier posts (To Pick or not To Pick and Just Pick 'em Already) on when to pick grapes for some advice on harvesting at the proper time.