RED MC is the only tool that can externally edit roster files. As noted a few times in its release and support topic, your security software (both in-built Windows and any third party solutions) will flag it with a false positive, due to leftover code from the now-unused license system. Browsers may also flag it as unsafe, so add exceptions/choose to keep files as necessary. It also comes with tutorials and guides to all the fields you can edit, which can be accessed from a menu.
I would suggest beginning with the last official roster update as the base. It'll no longer come through since the servers are offline, but you can still download it here:
Official NBA 2K14 Roster & Online Data UpdateOnce that's in place, save the roster. You'll find this save file in your AppData\Roaming\NBA 2K14 folder, which is where you can open it with RED MC to edit it as necessary.
My advice when it comes to roster modding, going right back to the heyday of the NBA Live modding scene, has been to do as much in-game as possible (moving players around, editing what's accessible in Create/Edit Player, etc.) and then only using the external editor (RED MC in this case) to change what can't be accessed in-game. That way, you'll avoid making any errors on simple tasks that might be tough to track down later.
The game has a Create-a-Player limit of 200. You can obviously overwrite original players that you don't need, which is often advisable for major roster projects such as total conversions and current roster updates over a decade later.
I think those are some key basics when you're starting out. Obviously, good modding practices such as keeping backups (and a few staggered backups so you don't lose too much work if you go wrong) is a good idea, but I'm guessing since you already have experience modding other games, you're probably aware of stuff like that.