I build my stuff in a model program around the 0,0,0 point. Build it at one to one scale. then I export x files out in pieces that fit together and use the same textures. Walls and stairs, windows and doors separate for scripting later. Roofs and ceilings, Furniture, depends on what I am doing with it.
If I want to use something from a model pack, first thing I do is make sure I have all the textures in .dds format. next thing is I put the textures, x file, and the FPE file and .bmp file in the same folder under the entitybank folder.
Then take an existing reloaded FPE file, Open it in notepad, then replace the info of your models, texture, and description in it. Save it as file name for model pack entity. Run reloaded and place your new reloaded entity in the map for testing.
Check that the textures are working, and that the desired collision you want is working.
So weather it is a custom model you are creating or a model from any model packs that are not animated at the moment, this is the way to get it into reloaded. Once you get the hang of it, it should take only a couple of minutes per model to get them Reloaded ready.