Well, you could weigh them and add them individually to water, or weight them and mix them together thoroughly dry, and then add a weighed amount of the mixture to water. The concentration is up to you, limited only by the solubility of the salts, but they are all quite soluble. As I mentioned in the quote you replied to, the mixture is 74.5% nitrate by weight, so the PPM in solution will be how ever many milligrams of total salts you added per liter of water you added it to. You'll have to work out the amount to add depending upon the tank volume.
For example, let's say that you took the proportions I gave you above to create a mixture of a total of 100 grams, and dissolved that in enough water to make a total of 1 liter of solution. That would mean you used 79.1 grams of sodium nitrate, 15.5 grams of magnesium nitrate, 3.3 grams of calcium nitrate, and 2.0 grams of potassium nitrate. The resulting liter of solution would contain 74,500 PPM of nitrate. Now, let's say you wanted to add 1 PPM nitrate to a 100 gallon tank. 100 gallons = 378.5 liters, so you would need 378.5 / 74,500 liters = 5.1 mL of that solution to get your 1 PPM nitrate boost.
Please note that the numbers I cite above all assume anhydrous salts are being used. Magnesium nitrate and calcium nitrate are hygroscopic salts, and the anhydrous versions will tend to turn into the hydrated versions in air, which can make it difficult to weigh them accurately. Plus, the amount needed would have to be adjusted in proportion to their hydrated molecular weight vs. the anhydrous molecular weight, e.g., if you use magnesium nitrate hexahydrate (MW 256.41) instead of anhydrous magnesium nitrate (MW 148.32), then you would need 256.41 / 148.32 as much, so in the example above you would use 26.8 grams instead of 15.5 grams.
On a practical level, I'd just stick with Randy's original suggestion to just use sodium nitrate, since it is not hygroscopic. But I did the math for you about the balanced cation version he suggested, since you asked. If you were to just use sodium nitrate, then it is 73% nitrate, so if you were to dissolve 100 grams in enough water to make up 1 liter of solution, then it would take 378.5 / 73,000 liters = about 5.2 mL instead to add 1 PPM nitrate to a 100 gallon tank.
