Level design is a complex beast, but since you seem new to it, I'll give you the basics of design.
first, select from your segments. pick segments that will match your level's setting
now build the first room/section of your map with these segments
once you've done that, add entities to this section until it looks well decorated and lived-in (or abandoned if that's what you're going for)
just keep going room by room from here. It's best to keep levels enclosed and use as many segments as you can even outdoors to stop other parts of the map from being rendered.