An excerpt from my workshop manual:

To make potassium oxalate from raw materials:

(You may want to do this outside or at least with good ventilation as the process gives off CO2, some heat and a significant amount of bubbles.)

To make 2 Liters:

1. Start with 1100 ml cold distilled water in a large container

2. Add 450 grams of Potassium Carbonate mono with stirring until dissolved. This will cause the solution to heat up.

3. SLOWLY add 400 gm of oxalic acid to the potassium carbonate solution with stirring, waiting each time you add the acid until bubbling stops. Continue until all of the oxalic acid has been added.

4. Add water to bring total volume to 2 liters. Check the pH at this point. It should be near 7.0 (neutral). Now add more oxalic acid in small amounts and keep checking the pH. When you get the pH to about 6.0, you’re done.

The pH of potassium oxalate is important. It should be used in an acidic state. This should be checked periodically with a pH meter or narrow-range pH paper. If the pH rises above 7.0 (i.e. alkaline) it can be adjusted by the addition of small amounts of oxalic acid. Add the oxalic acid powder a little at a time and keep rechecking the pH. I usually add oxalic until the pH reaches about 6.0. Filter your developer once in while and maintain the pH and you can use it for a very, very long time.