- Ensure you are subscribed or had manually installed the required mods/libs.
- Load all required mods in the mod manager

From here you can either create a new mod, or edit the ones loaded. I suggest creating a new mod because the mods you edit may get updates and you will lose your changes when update.

Either you decide to create a mod or edit an existing one the next step is create an appearance preset.

Once created you can start selecting the properties you want the unit to wear.
You can customize almost everything, choose the body, head, pants, etc, keep trying stuff until you are satisfied, in any case you can always create more presets to change later or just because.

Sometimes you can find items that are in the mod or in the vanilla game but are not showing in the combobox, in this case you can type the name of the entity you want. You can also type some other asset in any of the slot you want. It's not rare the case I use the "Hat" spot to wear an extra bag, lbe or anything I have in mind. If the asset looks weird you can try changing the "Origin" spot, again, you can manually type the origin you want in case it's not showing in the combobox.
Also you can try tweak the offset values to fit some asset that looks a little bit off.

You can click on "View in Anim Metadata Editor" to preview your creation.

When you are done messing around, you can either replace a merc appearance preset by saving the appearance you want with the name of the merc, there are others ways to do so but if you need this guide you probably will have trouble doing so, so just save the appearance preset with the name of the merc you want to replace.

In the case of IMPs the IDs are IMP_male_01, IMP_male_02, IMP_male_03, IMP_female_01, IMP_female_02, IMP_female_03.

Save the mod and you are good to go. GLHF.