Citric acid or crushed up vitamin C tablets help H dissolve into the water. You want to slowly add just enough to fully dissolve the powder. That is usually more important with tar, whereas powder usually dissolves a little better on its own (depending on how it is cut).
Step by step instructions for cooking and shooting:
1) Add H to "dish" that you can heat from below (a spoon or a test tube works well).
2) Draw up some CLEAN water (distilled or at least sealed bottled) with your syringe.
3) Add just enough water to your "dish" to cook your product. For a full G, you probably need atleast 6ml of water. You can add more while heating if necessary.
4) Add citric acid. #4 powdered H doesn't need any citric, #3 requires about 15% citric, and tar needs about 30% citric in order to dissolve (0.3g for every 1g of H). China white should be fairly clean, so your mileage may vary.
5) Heat your "dish" from below, with a soft-flame lighter held 2-4 inches under it. You want the water to barely start bubbling. If you are using a spoon, place the handle on the counter, bowl hanging over the edge, and place a book or something on the handle of the spoon. Now you can heat it one-handed.
6) When it looks mostly dissolved, give it a quick stir with the needle of the syringe. You can shoot a little more water in there if necessary. Dissolve as much as you can, but the water will likely stay a little cloudy from whatever they cut the powder with.
7) Pull a small piece of cotton off of a cotton ball, "fluff" it a bit (make sure it is loose and not a solid wad of cotton) and toss it in the soup.
8) Draw the soup up into an empty syringe (can be the same one you used for water and stirring), making sure you are drawing it through the cotton. This filters out small particles, which can damage your veins.
9) Hold the syringe with the needle facing up, flick it near the bottom of the syringe a couple times, and then push just enough to get a tiny drop of liquid to come out of the needle. This gets any air bubbles out.
10) Tourniquet above injection site, smack your arm a couple times to make veins easier locate, and wipe your injection site with an alcohol or iodine wipe. Infections and abscesses SUCK if you survive.
11) Enter the vein at a shallow angle (syringe should be roughly 30 degree angle from your arm), so that you don't accidentally go through. You can make sure that you are in the vein by pulling back a little on back of the syringe, which should easily draw up a little blood.
12) Push your dose, then remove the tourniquet. If using a syringe pump, you will have IV style tubing, and will need to remove the tourniquet right after inserting the needle and taping it in place.
You may need a higher than average volume of liquid for your setup, because the tubing on a pump would need to be flushed. Never used a pump myself, But I think a 10ml syringe is about right for a full G.