Short but complete answer: Step 1: Read the included documentation about the gimbal. Step 2: Attach the included counterweights as instructed there. Done.
Sneak preview: The Solo gimbal comes with a little black box containing some round counterweights. The largest two weights are marked "Hero 3+" and "6G" (in tiny print, engraved on the flat side). Attach both weights to the gimbal as shown in the included documentation. This will balance your GoPro in the gimbal as long as you don't attach any additional filters to its lens. ALSO you must select "Hero 3+" in the settings of the Solo app.
Now be sure to read ALL the Solo documentation if you want to fly it more than once.