How do you download the mod?
Just download it or is there something else that I need to do on my smartphone?
This is what I do on my phone, and it works (downloading Total Commander is very recommended, but I think your phone's original file explorer should be able to do the trick anyway):
1 - After downloading the mod, go to your app storage (mine is internal storage, but you could use an external SD card to install apps)
2 - Now go to Android/data/monstergirldreams.program/files/
3 - In there, create a folder named "Mods", and other named "game". Inside "game", create another "Mods" folder. Mods/ will be used for the mods' images, while game/Mods/ will be used by the rest of the mods' data.
4 - To make things easier for you, do the same as me: grab a mod's folder, put it in game/Mods/ (WITH ALL ITS CONTENT), then copy such mod's folder to Mods/. That way you don't have to create all the folder trees on your own, but if you're having storage problems, keeping only the paths to the mods' images in Mods/, and deleting the images' folder of every mod in game/Mods/, will help you save the maximum amount of storage you can.
5 - Make sure that you load a save in town, in case you could run into any errors. Also, when you load a game after adding/removing a mod, the first action you do on the game screen (like, for example, go adventuring or enter the church) will take a long time to perform, I guess it's because the game needs to load/unload a mod's content as soon as it detects a change in the mods list, which (I think) is checked at every valid click you do. So don't worry about your game "freezing", it should unfreeze in less than half a minute.
Okay, I think that's all you have to know about installing mods. At least, it's all I needed to know, but do any doubts pop up, I'll try to check them out.