The Roundabout may not require it, but the most popular carseat for aircraft use, the Cosco Scenera, does. You won't get it into position without raising the armrest.
The easier way to get the carseat on to the plane is to carry it upside-down and sideways -- think of the shape of the number 7. It that position you'll whack fewer people with it as you walk down the aisle.
As a general rule, it will be easier for a man to carry the seat onboard, but easier for a woman to install it. Being shorter with smaller hands makes installation in tight quarters easier.
To install it, first lift the armrest, then recline the aircraft seat fully back. Next, extend the aircraft seat belt out almost as far as it will go, leaving only a couple of inches of grab space for the tab end. Position the carseat in the proper direction and thread the seatbelt where it needs to go, then turn the seatbelt's buckle one-half twist before latching it. Set the carseat fully into position and pull up on the seatbelt until it is snug. Lastly, push the recline button and pull the aircraft seatback back to full upright position. (Installing with the seat reclined and then pulling it upright pulls out about 3 inches of "cushion-squish-slack" in the seatbelt; this is a substitute for kneeling in the carseat as you would in an automobile.)
Flipping the buckle is important because the seatbelt buckle on an aircraft is a gate-latch design. It is next to impossible to easily open the latch if the top of the buckle is flush against the shell of the carseat; you want it to end up facing away from the shell. If in the end with your particular seat the buckle ends up under the upholstery making a lump against the child's back, then you should ring the call bell and ask the FA for a seatbelt extender. Then you can shorten the main seatbelt and use the extension to "bridge" the middle of the carseat between the two seatbelt buckles.