How to make frosting from scratch? Mothers Day..?

Tres leches doesnt really need frosting it's sweet enough. But if you must just beat half a stick of softened butter with two cups of powdered sugar and enough milk to give it the right constitency. You can pour it into a plastic bag and snip of the end to make a piping bag. You can find videos of it online. Or you can just spread it onto the cake using a butter knife.