Hi and welcome! We have our Hope You Can Cling To challenges happening right now, and we have an Australian partner this year, so I hope you can join us. All the info is here: Hope You Can Cling To Challenge Forum - Splitcoaststampers
Okay - I normally think of a card insert as the lining that you write on inside the card - so if you're using a dark colored cardstock, you might want to make a piece of vanilla or white cardstock either the same size or 1/4" smaller for the inside so you can write on it.
However, a lot of people do make a piece of cardstock to cover embellishments - just make it the same size as the card front and it helps protect it even more if you crimp the cardstock with a paper crimper so it sticks up a bit.
I hope that helps!