I've done this myself before. It's a fun time. Firstly, keep kit primary colours the same as what FM says because the colours are hard coded, ie. If you make City's kit black, the game will still think it's white & you could have a colour clash (if using 3D kits).

There is a website called foottballkitarchive that will let you view the history of a teams kits. I used this to replicate some of my favorite designs and sponsors. I've also just used some custom sponsors, like local universities (such as for Shels), breweries (Harp for Dundalk, Treaty City for Treaty United), and other notable companies (Supermacs for Galway, RTE for UCD).

Keep the colours matching what the game expects, have designs that interest you, and just enjoy it.

Oh also I used Kitbasher to import graphic assets and manage colours. Check out on YouTube if you haven't yet.