You don’t need a buffer just pour them in carefully in the desired pattern. You can also tilt the vase while pouring it in to get layers in angles. I always tap it just a little bit to help settle each layer before pouring the next onto it that way it ensures that the layer levels out some kind of like cake batter in a cake pan. If you are doing your own colored sand you have to make sure that it is very very dry before you attempt to layer it otherwise the colors will bleed.

ETA. If you want one color to run into the other like a design you would just take a thin object like a kebab skewer and poke it through one layer into the color below it along the perimeter of the inside of the vase. This affect can look really cool and is fun.