Beat in butter by the tablespoon using a medium speed. The butter will deflate the frosting a bit. After all of the butter is added, turn the speed up to high. The whole process will take about 10-15 minutes, but you will begin to see the buttercream go through stages after all of the butter is added. First it will deflate and become soupy, then thicken, then curdle, then thicken to the final stage. If, for some reason your buttercream does not progress from the “soupy” stage (typically due to adding the butter too quickly or the butter/meringue being too warm), simply place your work bowl in the fridge for 7-10 minutes before whipping again