Instructions
Choose a historical time period or specific war to simulate with your mod. Research the era, focusing on military equipment, weapons, armor and uniforms. Study battle tactics and familiarize yourself with the major battles that took place during the time frame of your mod.
Choose a game engine on which to base your mod. If you've modded or played a particular moddable game in the past, use it. If its modding capabilities are insufficient for your purposes, you may prefer to try a different game. The games of the Half-Life and Quake series are popular for mods set in recent history, while Mount &Blade is better for medieval and ancient settings. The Civilization games are a good choice if you're looking to create a mod that simulates an entire culture rather than a specific military conflict.
Learn all you can about modding with the game you've selected. Most of these games have a wealth of free tutorials to help you get started and a large community of fellow modders to give you tips.
Begin work on your mod by creating historically accurate models for the weapons and units involved. Also focus on coding scripts to alter the gameplay and making textures for both the models and the user interface.
Ask for help from the gaming community, but not until you've got a lot done on your own. If you have finished work to show, it'll be much easier to recruit teammates. If you're not very artistic, find someone to model and texture for you. If coding isn't a strong suit, get a programmer.
Release both early and often by routinely offering your historical mod to the community for download. Not only will this quickly build your fan base, it also allows you to find errors faster than any play testing on your own can. Incorporate community suggestions for new features or ways to heighten historical accuracy in your next version.