The recipe for red bean filling is red beans, white sugar, and water. The specific steps are as follows:

1. Wash the red beans, add water, and soak for a day.

2. Put water in the pot, pour in the foamed red beans, bring to a boil over high heat, then simmer over medium heat until the red beans become soft.

3. After the red beans are cooked, separate the red bean water from the red beans, pour the cooked red beans into the pan, add white sugar, press with a spatula, and stir-fry until the sugar is completely melted.

4. Put the prepared red bean filling into a plate and serve after cooling.