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Install Heroes 3, the English version of WoG 3.58f, and Russian fonts (if Heroes 3 was a Russian version). Install the mod on a similar clean installation.
1. First, install new_objects_patch_ver_1_8.wog (a separate patch for the map editor - new objects).
2. Then, install commanders-gods-small.wog and INSTALL.wog (the mod files themselves, added to the game's resources).
3. Place the contents of the Data folder (additional mod resources) directly into the Heroes' Data folder, and place the map, 300.h3m, into the Maps folder.
4. 300restore.wog is a file that undoes the changes made in step 2, i.e., it's the mod's uninstaller. Perhaps you installed it without realizing it. If this is the case, repeat step 2 (rename the *.wdn files to *.wog in the UPDATE folder and patch the game again).
5. If you're not comfortable with English, it's best to wait for the Russian localization that Aleee promised. Since the game rules in this scenario differ significantly from the usual ones, it will be difficult to play.
P.S. Before starting the scenario, set the game difficulty to 200%, and then choose the difficulty level when loading the map through the mod menu.