Well, first make sure your comics are in a compatible format. Then, in ComicFlow, go to the settings or menu. There should be an option like 'add comics' or 'import files'. Follow the prompts and browse to the location where your comics are saved on your iPad to add them.
It's quite simple. Just open the ComicFlow app, look for the 'import' or 'add' option, and then select the comics you want to add from your iPad's storage.