A few tips:
The carseat normally must be installed in the window position. (It also cannot be placed in the rows directly in front of or behind the exit row per FAA rules.) How tall is your DH? On most aircraft, it will be MUCH easier for a shorter-height woman to install a carseat, because you are working under the overhead bin. (Also, women usually have smaller hands.) So, if you want to let the kid work off some energy and board late, it is generally better to let Daddy stay in the terminal with him while Mommy installs the seat.
When carrying the seat onto the plane, it is best to carry it upside-down and sideways (think of the profile of the number 7), this way you can see over the top of it and won't bonk anyone sitting in an aisle seat with it.
This one is important: unlike car seat belts, airline belts latch in the middle. What that means when installing the seat is that you will end up with the lift-latch of the buckle facing toward the hard back shell of the carseat, which makes it REALLY difficult to unlatch when you want to leave the plane. To make it easier, extend the belt adjustment to almost the largest length, then before latching the belt, twist the buckle-end of the aircraft seatbelt 1/2 turn, so that the lift-latch faces toward the back of the aircraft seat and away from the carseat shell, latch the seatbelt, and then pull the loose end to tighten down the seat. If you forget, kneeling in the carseat will usually free enough tension to let you open the latch to remove the carseat. Best technique to get the seat tight is to recline the aircraft seatback while you install it, then bring the seatback up again against it, which will snug it quite tight, but you won't be able to do that on Allegiant, as their seats do not recline. Again, your best alternative method is to kneel in the carseat to press it down. (NOTE that if you find on landing that you cannot get your hand far enough behind the carseat to unlatch the belt and remove the carseat, then wait until others have left the plane, go around to the row behind, and reach up between the seatback and the seat pan cushion to reach the latch.)
Even though the carseat will be a narrower one, it's likely that the armrest next to the carseat will not lower back down. Normally that is a problem, but as long as the carseat is solidly butted against it in the up position and strapped in tightly, the FAs should allow it with no difficulty. (The issue with having armrests down during takeoff and landing is so that they don't come slamming down and hit anyone. If they are solidly braced in the up position by a solid object like a strapped-down carseat, then there is no risk of them slamming down.)
Oh, one more thing: A seat with an integral 5-point harness that the manufacturer says can be converted to booster use by removing the integral harness does NOT meet the FAA definition of a "booster seat" as long as the internal harness is in place. This is important to understand because the airline will tell you that booster seat use is not allowed on aircraft. So if the packaging of a 5-point harness seat uses the word booster in some way, don't worry. (For a seat to meet the FAA definition of "booster", it must lack an internal harness and fully depend upon the vehicle's lap-shoulder belts to keep the child in place. That's the reason the FAA doesn't allow them to be used on planes, because aircraft seatbelts are lap-only.)
Also, expect that you may get pushback from FAs about using the seat on the aircraft, and especially if you decide to install it rear-facing, because not many people do that. However, on a US carrier, you are entitled by law to use an air-certified carseat in a paid-for seat space if you wish to do so. You also may be asked to show the FAA approval sticker to prove that the seat is OK to use on a plane. This is a bit of a trick question, because the official sticker doesn't mention the FAA. You'll find the sticker somewhere on the side of the seat's shell, and it will read (in RED lettering): This restraint is certified for use in motor vehicles and aircraft.
