Nothing is really simple. I started with the usual B'Mann Porter 0-4-0. I decided to convert it to a Forney type by making my own boiler, adding fittings and making a fuel/water bunker.
That lasted for a while until I fell in love with the Arizona Copper 0-4-0 (Backwoods Miniatures makes a great conversion kit, by the way). So I changed back to the original boiler and fittings, kept the fuel/water bunker, and added an open cab.
All went well until the original mechanics packed it in. So I replaced them with those from a B'Mann 0-6-0 saddle tank, stripped the ST boiler, replaced the fittings with those from the Porter conversion and added its open cab and fuel/water bunker.
There you go! Probably more information than you wanted!