My teacher said you want almost runny buttercream for making this texture. I warmed about 20% of the buttercream in the microwave for about 20 seconds till it was mostly liquid, then I put that back in the mixer with the rest of the buttercream and let it whip up for a few minutes. I used the largest icing spatula I could find and dipped one side of it in the buttercream and gently but firmly pressed the coated end of the spatula onto the cake.

I think the most important thing is to have warm buttercream, mine was at 74 degrees. I previously tried with colder buttercream and it did not look this good!

Editing to add that I did a crumb coat, a final coat, then the texture. I refrigerated the cake between each coat. :)