If you buy your books from the Kobo store, you don't have to do a thing; the E-reader will recognize the existence of the Micro SD card and start using it automatically when the main memory is full. You don't have to do a thing; the E-reader will manage its own resources.
If you get books from other sources, I recommend the free program
Calibre. It will give you the choice of whether to put the books into main memory or on the Micro SD card.
By the way, on most Kobo models the main memory IS a Micro SD card -- just one that happens to be hidden under the back cover plate. In other words, you have two now, and there's no particular advantage to having books on one or the other.