It's not difficult once you know how, but the smaller you are, the easier it will be, because there isn't much maneuvering room. Generally women can do it more easily than men.
Here you go: 1] To get the carseat onto the plane without whacking people, carry it upside down and sideways (think of the number 7). 2] Ask the FA for a seatbelt extension as you get on the plane. 3] The seat must be in the window position. Set it down and recline the aircraft seat as far back as it will go. 4] Attach the extender, thread the belt through the carseat, and buckle it, with the extender opened as loose as possible, and the buckle flipped a half-turn. 4] Put the carseat in the right position (rear-facing in your case), and pull up on the loose end of the belt until it is fully seated. The buckle will probably disappear behind the carseat. 5] Lastly, pull the aircraft seat into the full upright position again; this will take up the remaining slack.
Be prepared for the possibiility of being questioned by the FA about the rear-facing seat. They don't like them because they keep the seat in front of it from reclining, but you *are* supposed to install it according to the mfr. instructions.