Make a map level. no special scripting, just buildings and trees and stuff. Door scripts will work. Place weapon pickups around the scene.
Save your map file.
Initialize multi player and create a workshop. Once it is uploaded, then you can set it for availability in the lobby for people to join and download and place.
The map you want to host, needs to reside in the root of the mapbank folder. You may have a folder in there called legacymaps. It has some MP maps in there. These can be uploaded as hosting also. You will need to copy them into the root mapbank folder, to be able to see them in the host a level window.
Intel(R) Core(TM) i5-8400 CPU @ 2.81GHz, 32GB RAM, Nvidia gtx1050ti 4gb, Windows 10 Home 64bit, Screen resolution 1920 x 1080. System Passmark 3774