First, you should start by guessing what "missing source image sprite/tileset" means. But surely you already know.
Second, you should read the Lua API whenever you don't know how to do something; otherwise you will not learn. You will find the answer to your question in the Overview of the Lua API for the door entity, in the following link:http://www.solarus-games.org/doc/1.5/lua_api_door.html
Keep a link to the Solarus Lua API in your browser (that is what I do) because you will be constantly using it whenever you forget something, or just to check the syntax of some functions, etc. Before asking in the forums, one must try to find answers there (if you don't find your solution there, then ask in the forum).
Third, I think you should forget, at least temporarily, about making a guide if you cannot make easy things work. Be realistic, one must be an expert to write a guide about something. I say this for your own sake. For now, focus on learning things, experimenting with the engine. Some tutorials of pure Lua may help you a lot too (there are many in the internet). If you become an expert of the Solarus editor, then you may be able to write a nice guide.
And my last advice: the best way to learn is to learn from the master (Christopho in this case). That means you should watch all his video tutorials on youtube and study many scripts of his games until you understand most of the code, and then start experimenting by making small changes. (Avoid the hud and dialog box scripts if you are still beginning to learn Lua, because these are more complicated, but try to understand all the other basic scripts.)
PS: You don't have to follow my advices if you don't want to. This is just my opinion. I don't intend to be rude.