Choose a design idiom for your fireplace hearth that blends with the surrounding architecture of your home and its furnishings. Decide between sleek, crisp and modern; warm, traditional trimmed molding; or something in between. Also choose a position, a traditional look is lower to the ground, or a more modern placement is higher up the wall.
Build out the hearth in the design and style of choice with the interior firebox opening matching the specifications of your ventless unit with between 1/2 to 1 inch of extra space, which the fireplace unit frame will mask, or according to your manufacturer's directions if different.
Slide the firebox unit evenly into the hearth opening about halfway. Pull the gas connection or electric hookup through the side of the unit and connect to the power source. Slide the fireplace unit all the way into the hearth opening, making sure it is level.
Trim the fireplace hearth with tile, marble, slate, wood trim molding or whatever is in keeping with the chosen design. Ensure that any finishing of the hearth does not obstruct the vents on the front face of the firebox.
Assemble fireplace logs in the placement and pattern outlined in the manufacturer's specifications so that air flow around the logs is good, the flames are evenly spread in the firebox and the fireplace operates smoothly.