How do I build an environment from a blank world to build an environment, about the organization inside the xml, here the official documentation, I can not understand, the example on github is not a step by step, given my foundation, it is really confused. 我怎么构建从空白世界一个个物体来构建环境，关于xml里面的组织，这里官方文档，我看不懂，github上的例子不是一步步来的，鉴于我的基础，实在困惑.