This is the procedure that I was given by Carl Weese a few years ago:

Put two liters of cold water in a large bucket. Stir in 600 grams of oxalic.
Don't worry about dissolving. Measure an equal amount of potassium hydroxide and very slowly add. The mixture will bubble up lots of CO2 and get real hot when the second ingredient is added. Next, use Ph test paper to
check the solution. You want dead neutral. Add tiny amounts of either acid
or alkalie until it's neutral. This is the only tricky part--amazingly small
amounts shift the solution from acid to base and it will take a while to
find exact neutral.

Let me add a few notes to the description above. This will make a solution that is about 25%. I believe saturation is around 30%, so you may run into issues wit material falling out if you get too close. Just add more water. In most cases, the saturation of PO is not critical for developing, so this should not be an issue.

Definately use COLD water, as the reaction makes a lot of heat. Also, use distilled water to avoid contaminants. For pt/pd, I make the solution pH neutral, and then add a little more oxalic, because the pt/pd chemicals are quite adverse to a basic solution, and I want to make sure that the error is on the side of acidic.

Also note that The Chemistry Store has great prices on citric acid, sodium sulfite, and a handfull of other useful alt chemicals.