The bolts holding the jack to the frame will provide your ground. Mine is wired like this. From the fuse (30amp) block to the jack hot wire. However my fuse block is hard to get to and I would go with an inline fuse near the jack for ease of changing (it's on the list). This wire still need to terminate on the fused side of the block in case it shorts to the frame between the block and the jack. Use a larger fuse in the block so the jack fuse goes first.
Very important that you follow the calibration instructions. When hooking and unhooking I have the trailer plugged into the truck and the truck running so I don't use the battery. These things draw alot of amps and the small gauge wire they come with doesn't help. If you don't have a wire going to the block now use at least #12 marine grade straned wire. At least means #12 or heavier (lower numbers are heavier 10 8 ). For an extra dollar you'll have a top notch setup.
Your instructions will tell you what size fuse to use.