The best thing to do, besides writting your own editor is to use someone elses, like WorldCraft. www.flipcode.com has an page talking about using that editor for your own project.

-Nathan Strandberg

Look into d3d retained mode when making an editor for the first time. You''ll find mesh functions, picking functions, level of detail functions and others that help out. I found out that using D3DX basic object primitive functions like box, cylinder, etc. limit you in what you can do with them. Think of creating a face made out of mesh then do collision detection between mesh and object and remove polys from mesh or split intersected triangles into smaller triangles. Also check out Genesis3D editor source code for Constructive Solid Geometry and find out how WildTangent did it, or check out CrystalSpace code or how about source to Quake editors?.