Add a tilemap object to your scene
it will load a default tilemap bg
use the open button to choose your own tilemap
on the properties panel on the left side of the screen there are options to see how big each of your tiles are etc, so it knows how to break them up.
No pick a tile by clicking on it from the tilemap menu (might be hidden as a tab next to the objects window depending on your screen layout.
pick the pencil icon and chooose a tile in the window below and start drawing in the game play area,
you can also pick the rentancle tool and choose an area of tiles in the tilemap window eg: a 3x3 block then click and drag to draw a 9 patch rectangle of tiles