ok what you have to is at the top on your tool bar under tools (don't go into tools) there should be three layers and the bottom one should be highlighted, the one on the right should have the middle one highlighted and the one next to that should have the top one highlighted.....so you choose the one with the bottom one highlighted and the select your grass and lay it down...then select the one next to it which should have the middle layer highlighted, click on that one.....then select yur tree or whatever and set it where you want.....it should be fine...to check your work you can click on the cube icon to the left of the pencil on the toolbar.....does that help you any?