Most would start with replacing an existing character with something else, and general moveset and character alteration. Adding a new character often adds a whole other dimension of complexity beyond understanding what the character's "stats"/movelist works like.
The simplest means of adding another character tends to be something like making an alternative skin/move set for a character you select with a button combo on selection.
Adding a new character "properly" tends to involve understanding how new characters are designated (hopefully there is "space" in the selection -- if there are only a few bits the number of characters, including bosses and such that might be designated as that*, then the devs could have filled up every slot and you then get to expand the selection parameters), their animations, their moves, any stats if the game does differ here and also get the game to read it (can be a matter of extra lines on a table, can also be a whole other nightmare if it is all programmed in with unique per character quirks).
If you are porting graphics across then you also get to figure out any resizing (if any is needed, MK2 to 3 should be OK compared to 1 to others), create any missing animations (can't remember what went for animalities, babalities, friendships and such for 2)
*finding and playing with this designator then often being a good way to create play as boss, play as hidden/locked character, play as same character (some lock it because they can't be bothered to have alt colours), and play as glitch characters cheats and other unlock hacks. Fairly doable as a cheat search, though will depend whether the game finalises selection when it is all locked in or when it is hovering over a character so don't necessarily expect what is on the selection screen cursor to be what the game sends.
As far as tutorials... for any game that does not feature an internal custom fighter creation option then it is probably going to be one of those "if you have to ask (or at the very least could not dash out something similar to what I wrote above for the main pitfalls and approaches) then you are not ready" type things, and I hate doing that.