Setting up this entity is actually quite simple!
Once you place the spawner, open up its properties and set the following keyvalues:
- You can leave the name blank if you plan to operate all spawners in the map together; otherwise give it a unique name.
- A life time of 0 (seconds) will allow the skeletons to live until they are killed.
- Setting the skeleton count will cap the number of active skeletons for that spawner.
- Setting infinite spawn to Yes will cause the spawner to replace skeletons as they are killed.
- There are 2 options for skeleton type, Normal and King. Both of them are the green, hostile-to-both-teams variety.
After that's set up, then you'll need to add a logic_auto entity. Navigate to the Outputs tab and add the output below. If you left the spawner unnamed, simply put tf_zombie_spawner as the targetname instead, and it will fire the output to all unnamed spawners (it will also be marked as broken in Hammer, but it still works).
Finally, once you compile the map and load it in TF2, type sv_cheats 1 into the console followed by nav_generate. This will create a nav mesh for your map and then reload it, allowing your skeletons to move around.