Open the editor with F2 and open activity edit mode with the symbol in the top bar of the editor window. Press edit activity. The activity includes all blueprints for your scene and defines events and goals for it.

Open the Blueprint Library (F6) and drag Blueprints into the scene. These will be their starting positions when the scene is loaded.

Add playable blueprints by adding controllable blueprints and selecting start as player. Press add standard pioneer to add a placeholder for the players set standard pioneer.

Save your changes to the scene by pressing save scene in the top. Share your scene from the scene library (F5) by dragging out the scene PNG or uploading it to the workshop.

See how to edit scenes here:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=654882847