You should have at least one zombie spawner per zone of your map

Creating the Spawner in Radiant

First, drag an actor\spawner_zm_factory_zombie from the entity browser into your map. 

Give it the following KVPs:

Key

Value

script_noteworthy

zombie_spawner

coop_count
{9999, 0, 0, 0}

count

9999

script_forcespawn

1

Risers

To create a riser location for a zombie to rise out of the ground, drag a script\struct from the entity browser into your map. Give it the following KVPs:

Key

Value

targetname

<zone_name>_spawners

script_noteworthy

riser_location

script_string
find_flesh

Windows

To create a window entrance for zombies, drag a misc\_prefab into your map. Select the following prefab: _prefabs/zm/zm_core/barricade_reciever_wood.map

Give your prefab the following KVP:

Key

Value

script_string

<entry_name>

Create a riser struct anywhere behind the window and give it the same exact KVP you gave to the window prefab.