First, come up with a basic idea.
Then, make a list of things that come to mind about that idea. If it's a survivor, think of what their gimmick or personality could be like.
If it's an item, think of what it could do, abilities, drawbacks, etc.

Now that you have some good ideas of what the thing should be like, start coding each attribute individually.
Coding an entire mod is quite a task, coding an item or character is easier, coding a single attribute of an item/character is nearly effortless. Break the mod down into small pieces, finish one piece at a time, assemble it all together at the end.

Also, try to avoid getting burnt out during projects like modding. Don't pressure yourself to finish the entire mod without stopping.