Realistically, there's only one proper way to texturize a 3D model, and for it you need the 3D model. You can paint on top of a texture, but it's a very time consuming job, and the quality is not as good. Ideally, you would use a program like Substance Painter. I believe Blender has a way to paint textures, but I'm not really sure. But in any case, you need the original 3D model. At this point, I'm not entirely sure if I should give away all my models to benefit modding. That is something I'll have to keep considering.
Now, despite that, I am giving away one single model, so you can use that one for texturing purposes, or for whatever you want really. I'm aware once you release something to the internet, it no longer is "yours". This generic model won't be useful to make textures for every model in the game, but at least will allow you to create your own characters, although in a more limited way.
I've gathered all 3D related modding files in this google drive folder. Download GenericDoru_LP.obj and use it in your texturing software of choice. You only need a diffuse texture and a normal map. I've made available a high poly version of this model as well to help you baking normal maps.
Get your textures in Unity. I created a new folder beforehand. The diffuse texture requires no setup, but the normal map does. Select the normal map texture, and in the Inspector, expand the Texture Type dropdown and select "Normal map". Then press "Apply".
Expand the Texture Swappings field and change its Size property to 1 to reveal two texture fields. You can put there your two custom textures.
Vanilla models have many times more than one material per model. A good way to ensure all materials are correctly overriden by your textures, is to change the Size property to 3.
Now, it's very likely that you are going to want to replace also the vanilla textures of the boots. Doru NPCs have removable footwear, and thus require an extra texture for them.
In this field, put textures made for the boots, repeating the same type of work you did to retexturize the model. Note, that if you are replacing the textures of the generic/naked doru, you won't need to follow this step, as they don't wear footwear.
After this, you can save your ModelContainer prefab, build your mod and test it in the game.