I'm trying to get the embossed effect of a diamond pattern or I think it's also called quilting on my buttercream cake without using a mat (don't have time to order). a detailed instruction/tutorial would be great.

Maybe you can use one of those triangle rulers and make lines at equal intervals (using the long side) first in one direction and then the other. You can use either your spatula/knife or even a piece of uncooked spaghetti to make your lines in the buttercream. Just line it up with the triangle. I hope I'm making sense.