Many years ago I was asked by the staff at Model Railroader magazine to build a 4' by 8' HO project railroad. The purpose of a project railroad is to document the building process. This helps a newcomer, or seasoned veteran, to build the railroad.
I documented the process in 2 how-to-do-it articles. These were published about 15 years ago in Model Railroader. The story has all the information you'll need to build this small layout or a larger railroad, based on a Southwest theme. Check it out, it's a free PDF download at www.mrscenery.com.
I also put up a link to a 9:31 mp4 video about building landforms using cardboard strips and plaster wrap. This is basic scenery building information. The download costs $5.95.