Once the cake layers have cooled, fill and frost as usual. If making a tall double barrel cake as we did, fill and stack the bottom two 6 inch cake layers. Push in 4 supports (we use bubble tea straws), spaced out, cut to the height of these stacked layers. Place a cake cardboard on top (trimmed down to size), and then stack on the next two filled 6 inch layers, so that you have one tall four layer cake.