Synthesis of potassium trioxalatocobaltate(III) trihydrate
It appears that Sørensen was the first to publish the synthesis of trioxalatocobaltate(III) trihydrate [1].
A summary written in English (and that should answer both of your questions) including chemical reactions can be found in the very first volume of Inorganic Syntheses1 [2, p. 37]:

C. POTASSIUM TRIOXALATOCOBALTIATE
$$\ce{K3[Co(C2O4)3]·3H2O}$$
$$\ce{H2C2O4 + CoCO3 → CoC2O4 + H2O + CO2↑}$$
$$\ce{2 CoC2O4 + 4 K2C2O4 + PbO2 + 4 HC2H3O2 → 2 K3[Co(C2O4)3] + 2 KC2H3O2 + Pb(C2H3O2)2 + 2 H2O}$$
Twenty-three and eight-tenths grams (0.2 mol) of cobalt carbonate is dissolved in a solution of 25.2 g. of oxalic acid $(\ce{H2C2O4·2H2O})$ and 73.7 g. of potassium oxalate $(\ce{K2CrO4·H2O})$ in 500 cc. of hot water.
When the solution has cooled to 40°C., while it is vigorously stirred, 23.9 g. of lead dioxide (see synthesis 16) is added slowly, followed by 25 ml. of glacial acetic acid added a drop at a time.
The stirring is continued for an hour, during which time the color changes from red to deep green.
After the unused lead dioxide is filtered out, the trioxalatocobaltiate is precipitated by the addition of 500 ml. of alcohol.
The material appears as emerald-green needles, which are sensitive to both light and heat; The yield is 70 g. (71 per cent).

Notes

Please disregard obsolete typography and notations used in this textbook such as full stops after unit symbols, as well as the lack of white space between the numerical value and a degree symbol (as seen in “40°C.”) and use of notations such as “cc.” (cubic centimeter).

References

Sörensen, S. P. L. Kritische Präparatenstudien. III. Darstellung von Einigen Kobaltidverbindungen. Z. Anorg. Chem. 1896, 11 (1), 1–5. https://doi.org/10/fh47nq. (in German)
Inorganic Syntheses; Booth, H. S., Ed.; John Wiley &amp; Sons, Inc.: Hoboken, NJ, USA, 1939; Vol. 1. ISBN:9780470131602, https://doi.org/10.1002/9780470132326.