There's lots of pictures at the source. This is roughly the recipe, reworded for simplicity:
Ingredients: (use vegan versions, obviously)
1 cup milk
5 tablespoons flour
1 teaspoon vanilla
1 cup sugar
1 cup butter
Put flour and milk in a saucepan and place over medium heat. Whisk as it heats up until it gets really thick. Allow to cool completely. Add vanilla.
Cream butter and sugar together. Add cooled flour/milk/vanilla mixture, and beat with electric beaters until texture resembles whipped cream.