How do I install a portable outboard?

Important installation guidelines:

Make sure you are not overpowering your boat,  Never install an outboard with more horsepower than the maximum rating on the boat's capacity plate.  Otherwise, it could cause severe instability.  If a boat doesn't have a capacity plate, consult the boat manufacturer.

It's important the outboard is installed on the centerline of the boat. If the outboard isn't in the center, the boat could be tough to steer and exhibit strange handling underway.
Some transoms have a dot or discreet mark indicating center, making the job a lot easier. If not, there are several ways to find the centerline, none too difficult or time consuming. For example, determine the transom's center by finding the centerline of the keel and drawing a line straight up to the top of the transom. Or level the boat, side to side, and drop a plumb bob from the transom to the keel's center, and mark the transom accordingly

Once centered on the transom, the clamp screws must be tightened to secure the engine on the transom. Turn the clamp screws evenly, so the clamping pressure is about the same for both clamp screws. Tighten them as much as you can without distorting the transom. Watch for soft transom material which would be an indication of an unsafe boat. Ideally the clamp screw handles can fold over and meet in the middle, to allow padlocking them together. Locking the handles together not only deters those who would be inclined to abscond with the customer's new outboard, but also will prevent the clamps from rotating and loosening.
If the motor has a motor resistant cable attaching point, fasten a cable or chain from the outboard to the inside of the boat to prevent the outboard from parting company with the boat without notice in the event someone forgets to periodically check the clamps.

With the clamp screws tight, and the engine positioned vertically, check the engine height in relation to the bottom of the boat by placing a straightedge along the keel to the outboard. A good guideline is to have the anti-ventilation plate even with the bottom of the hull or slightly below the boat's bottom, maximum of 1 inch. If the motor is mounted too high it may not get cooling water at all speeds and the prop could ventilate. If it is mounted too deep in the water there will be excessive drag affecting performance plus extra parts in the water could create excessive amount of water spray which could enter the air intake.