Hi and welcome on videohive
Every item here has some instructions included on how to customize the template.
But, depending on the item you bought and its author, the quality of help files / tutorials differ. Some templates here are very well documented for Beginners, others have just very basic descriptions and leave you more or less alone with the software.
As every author can build his templates in his own way, there is no general tutorial for the templates here.
But you can always try to contact the author of the template for better instructions.
In my files, I always try to make it as easy as possible even for AE-newcomers. And I had very good feedback from AE Novices so far who hadn’t had problems with working with the files.
If you are interested in one of my items I can send you the tutorial to take a look at and see if it describes the whole process well enough