Start small. Try adding a few units to a nation. Check if it works. Then try making a new unit. If you aren't an artist, you probably won't have images to use in it, and I can send you some to get you started. Mainly sprites from the game, but I have edited some of them.
Write down the idea. Add things to it, remove some, try to scrabble little description in there. And check the pricing guidelines at
www.illwinter.com (click Mods, then modding guidelines).
If you have a playable version, and are sending it, and decide to add a tiny, simple thing, however small,
check that it works! I have made many a mistake when sending the "surely working" program/file onward without checking.
And after you have checked that one last small thing, send the mod here, and prepare to edit it. It takes skill to do something, anything, balanced. Illwinter team has added unbalanced pretenders/themes/things in patches, like the Vampire Queen (it was fixed afterwards). Also, you have to decide if you want to make it combatible with some other mods. As an example, some people prefer Conceptional Mods series' nations, and would like to add you nation into it. It would be some extra work, but could get your mod more users.
Try to enjoy it, but remember that creating is always work. If you can finish it, you have something to be proud of. Many mods just are never completed.