Combine 20 g of cane sugar and 100 mL of conc. nitric acid in a 500 mL Erlenmeyer flask, start heating on a hotplate until the reaction kicks off. After the reaction slows down start boiling it slowly, boil until about 30 mL of the solution remains in the flask. Pour the solution into a beaker, and boil to 15 mL, and add 15 mL of water, boil until 10 mL of the solution remains. Cool down slowly and filter. Recrystallize from hot water.
The reaction kicks off like this
Boiling the solution down to 30 mL
Water was added and the solution was boiled down to 15 mL
Filtration of the crystallized mass
Recrystallized pure oxalic acid dihydrate