As you do not give details where you are stuck I can only guess.
One option is you tried to save your lib assets to the system lib which is write protected for good reasons. (The next update would overwrite your changes) You need to make a personal library to hold your personal assets.
But you might be stuck at some other step. So here my tutorials:
- Tutorial: How to make a footprint in KiCad 5.1.x (From scratch)?
- Creating a new footprint library
- Tutorial: How to make a symbol (KiCad v5.1.x)
- Creating a new symbol library and a new symbol in KiCad 5
After you understood that process you might want to decide how you want to organize your personal libs. There is no one right answer as different workflows require different organization schemes.
And in case the question really is about how to get downloaded assets into KiCad: