As the title suggests I want to know how you make a dev kit for people to mod your game?
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The first question that should be asked is, Can this be done, short answer: Yes (Anything can be done given enough Time, Money, and Effort).
The practical answer for this: For most people, No, they don’t have enough Time, Money, and Effort to pull it off.
Assuming that you still wish to do this…
1st: Determine what kind of Mod(s) you wish to allow. Alteration of an existing level? Creation of an entirely new level? Allowing new weapons, new vehicles, new Foliage, new X? Allowing new animations? Allowing new sounds? Allowing new capabilities? new Materials? new UI? etc
2nd: Will this be single or mult-player? As this is going to simplify or add more complexity (Single VS. Multiplayer)
3rd: Is this modding effort going to break any part of your games security, and if so, how can you prevent that from happening.
Before moving forward on any part of the Mod Editor, I would think you would have wanted to think through the above 3 questions at a minimum, before starting to write the first line of code, for the Mod Editor that you will need to build, and then finally putting together the Mod Engine that you will need to have in order to support this effort.
Before anyone can really help you though, you will need to reduce the scope of your question, because as your question stands right now, one could either write a doctoral thesis, or another Sams How To in 24 Hours book!
I did find this link quickly at youtube though, that may help you to get started thinking about this ( I didn’t watch the video).
Hope this helps,