Base Mod used: https://www.nexusmods.com/skyrim/mods/82522/?
Required Mod to make Adoption Work: https://www.nexusmods.com/skyrim/mods/29249/?
1 - Start Creation Kit (CK), select Skyrim.esm, Hearthfire.esm (contains the adoption quest and references) and the mod with the NPC you want to change; wait until CK loads everything, then look for the NPC reference on the search field. In this example, I will use "Nono" since it is the NPC name reference; look for the NPC form and open the Actor Window.
2 - The first thing you want to do is to assign the correct voice type on the Traits tab, so Nono can have the dialogue for adoption (MaleChild/FemaleChild are the ones you want); it is a good idea to check if the height of the NPC is consistent with what you expect of a child size, so move to something around 0.75-0.90. Change the disposition Base to something higher to make the NPC like you from the beginning.
Optionally, you can make the NPC unique and Essential, so it can't be killed. If you set to protected, it can be killed by your hand only.
3 - Skip the Stats tab and mod to Factions; clear everything from the Factions and Ranks list, and then right-click on that field space and choose new; a window will pop up, text "adoptable" on the search field and select "BYOHRelationshipAdoptableFaction"
This is what makes her adoptable; in the same field you can add more factions for security: town faction to avoid violence from other NPC towards her (if the NPC is in Whiterun add TownWhiteRun Faction and CrimeFactionWhiterun).
4 - Technically it works already on a new game. But you want to make sure to remove everything from the NPC/follower than can conflict or may not be consistent with a child behavior. So on the next tab, Relationships, you want to clear the Relationship NPC field and add a new one similar to the next image, using right-click like in the Factions part.
On the AI Data tab, change everything to something you expect from a child and not from a follower (aggression, morality,...), same with the Inventory and Skills & Spells tabs.
5 - Time to test. Install the mod required above then launch the game; load a save, travel to Whiterun to find the NPC; place yourself in front of the NPC, open the console, click on NPC and type:
SetFactionRank xx004290 1
where xx is the 2-digit code of Hearthfire in your load order (possibly 02 or 03)
If you haven't adopt any child or have adopted 1 at most, this should be enough to adopt her; if not, continue for step 3.
3 - Again in console (no need to click on NPC for this codes), type:
setstage BYOHRelationshipAdoptable 0
This will reset the adopt quest, but you won't lose any current adopted child (at least in my game) and gain room for more.