cp is a mod in which you didn't have to change the whole content (unlike xnb, which replace the original assets) for it to work. you simply need to turn those xnb or png files to cp. don't forget to download the content patcher mod first for it to work! first, a cp file should have these three things.
make a folder, and inside it you should have these three things. first the asset folder. simply make a new folder for that, you should put your xnb or png file inside it. that's it for the asset folder, now you need to make the content and manifest.
the content and manifest must have JSON format, but you can definitely edit it easily through notepad. I'll teach you how to create it, open up your notepad app include your code. I'll explain it simply.
"Format": "1.3", // you should delete the // and the explanation next to it for this to work.
"Action": "Load", // see what the codes meant in content patcher tutoria page, this is just a simple code i usually use to replace the asset that i want.
"Target": "Animals/BabyGoat", // you should check your content folder for this to see the right path. for this example i want to change how the baby goat look. babygoat.xnb is inside the animal folder so write the target as Animals/BabyGoat.
"FromFile": "assets/BabyGoat.png" // remember the asset you put inside the asset folder? you should write the .png/.xnb according to what file you put there.
}, //don't forget the comma if you want to write another code in one.
"Target": "Animals/horse",
"FromFile": "assets/horse.png"
Don't forget to delete the // and explanation next to it so that it can work! to check if your code is right or there's something missing or not, go to smapi validator, remember that if you miss even only one comma, it won't work.
click save as and rename the notepad as content.json, change the format to ALL FILES! this is very important. Next is the manifest, it's not as significant as the content but you need it to work. You need to create it exactly like content file, here's the code, it wll be the same in every manifest file.
"Name": "Animal", // name of your mod
"Author": "Test", // name of the modder
"Version": "1.0.0", // it should be the latest, but usually from my experience, you should lower the version if you want to use mobile (it will only work if the mod where version isn't significant for example a grass recolour mod).
"Description": "New animals", // write whatever for this
"UniqueID": "Cute.animal", // unique id is for your mod write whatever just remember you need the dot.
"MinimumApiVersion": "2.5",
"UpdateKeys": [ "" ], //for nexus
"UniqueID": "Pathoschild.ContentPatcher"
that's it, you should do the same thing as the content before. the name of the notepad should be manifest.json and change it to all files! After this, you're done! if it isn't working, see the smapi log (when it loads the mod, you can see the black screen right? see what's wrong with your mod and change accordingly), you can also check again on smapi validator to see what's wrong.