For actual modding there is a complete section a couple of sub forums down from here see:
https://forums.totalwar.org/vb/forumdisplay.php?f=72
best way to find a specific tutorial is:
https://forums.totalwar.org/vb/local_links.php?catid=70

as for how to use and existing mod - hopefully they come with instructions!

If you want advice on which one to try, post what sort of thing you are looking for in this forum:
https://forums.totalwar.org/vb/forumdisplay.php?f=119