I don't have good advice on preparing sterile water unfortunately... I always buy it. As to the recommended dilution, Dr. Cathcart's paper said this:
The Stock Bottle of Sodium Ascorbate
Sterilize a 500 cc IV bottle along with a funnel, the rubber stopper, and a spoon. Then fill the bottle to the 300 cc line with sodium ascorbate fine crystals. (I weighed the sodium ascorbate out one time and 250 gm came up to the 300 cc line.)... Then add water for injection q.s. 500 cc. Shake up the bottle and if there is 1 mm of crystals left on the bottom, add 1 mm of water to the top. It turns out that sodium ascorbate is soluble to almost exactly a 50% concentration at room temperature...
Preparation of the IV Bottle
I recommend that the above stock bottle solution be added to lactated Ringer's such that 30 Gms (60 cc) to 60 Gms (120 cc) this be added to a quantity of lactated Ringer's sufficient to make 500 cc of the final solution to be injected IV. I had been using water for injection some time ago because this solution is several times hypertonic already and I did not want to add more tonicity. However, recently I have found that lactated Ringer's feels better to patients so I use that for the final dilution (not the stock solution described above.)
So, if you are using lactated Ringer's it appears Dr. Cathcart recommended no more than 60 grams of sodium ascorbate per 500 cc final solution. That is approximately what I have found, as well. I typically end up using no more than 100 grams per 1000 ml final solution, although according to Cathcart's instructions, you should be able to go as high as 120 grams.
The Riordan Clinic has the following recommendation:

That info is found on page 13 of "The Riordan Intravenous Vitamin C (IVC) Protocol" paper. It's a good paper to read for more info on using IVC properly.