add all the plants that you like and that go well together

create a rectangle with the values as described
i used a ruler to get a feeling of how big it will be 

place the plants on the rectangle so that they are different sizes and move in different directions
as a good control, you can lay these individual blocks on top of each other so that you have an idea of what the finished product will look like

choose your laser power

i used acrylic pens here 
please note that some things can be a bit delicate - be careful when painting before they break

glue all layers together and make sure that the bottom block is straight everywhere
this has the advantage that it can stand up well afterwards