Im working with a little scene and have come across a snag making my level. I've look through the manual and it gave me some hints but still striking out. Here are the steps I took, Im using GS 7. I made a house. I build a box, then hollowed for the house to fit into fairly square but larger of course. I aligned objects to the grid. I applied a sky texture, blue textures to walls and grass to ground. I try to compile and get a E003 and a concave warning, it will not build.

Could someone please direct me to instructions on how to properly build a level or add a 'skybox' or give me and the othes here a step by step? Much appreciated!