Greetings @NubianQueen!
Maybe I’m not understanding this well (apologies if so!) but I’ve never heard of tarot cards being read horizontally (on their side)- they are almost always placed vertically (and are either right side up or upside down). Perhaps this is something unique to your deck?
Are you perhaps talking about the Deck of Oppositions? It is the only deck I know that has both the “upright” and “reversed” versions of the card pictured.
Picture from Tarot Deck Review~ Tarot of Oppositions
If this (or a very similar deck) is what you’re working with, it is indeed a very unique deck and has unique ways to read it! I usually shuffle well and then read the picture that appears on top, although sometimes I read the “middle version” (the point between upright and reserved). Any way is fine, but it is important to specify your reading style before drawing and revealing the cards.
Like Amethyst said, the guidebook is really helpful in walking you through the methods and providing examples 
Blessed be and happy readings! 