You should draw an overhead view of the layout and plan out where the players spawn, what walls are where, maybe mark where props like trees, bushes, chairs, cabinets and potteries are, then figure out if want more than one floor for buildings or how big you want the outside to be and go on from there
Once you have the basic layout and geometry done, you can add in minor details like decals for dirt, blood, graffiti, and maybe some detail sprites for grass blades and flowers
The Valve Developer Wiki has some helpful articles about level creation on the Source Engine to give you some ideas how to do things if you're not sure how to do them
https://developer.valvesoftware.com/wiki/Category:Level_Design
