Read the instructions on the bottle. (/sarcasm)
But seriously, use the ratio described on the bottle to guide you. You should clean beforehand with whatever your normal cleaner is, then mix up some starsan solution.
If I'm not mistaken, it says it requires 20 seconds of wet contact to sanitize. So swirl it around and let it sit for a bit, then pour the starsan solution into whatever you need to sanitize next. DO NOT RINSE. The bubbles turn into yeast food.
If the solution becomes milky to the point of translucence rather than transparency, it has lost it's effectiveness and you need to mix up another batch.
I would think that a gallon of solution would get you pretty far in your sanitizing.
Any other questions?