Ive done it twice when my son was 10 months and 21 months. I was traveling alone with him and my daughter (age 7 on the first trip , 8 on the 2nd one). We had an A boarding pass each time. I highly recommend going to the back of the plane.
On the first trip, I went to the back of the plane because at the beginning of boarding most people sit up front so I had more room. Plus back there we could wait out everyone and have more room once we landed. On the 2nd trip, my daughter didn't wait for me to fold the stroller at the end of the walk-way. When a lady saw her get on the plane alone she instructed dd to sit up front. It is much easier to install a car seat in the back.
Hardest part was getting the car seat back into the bag after the flight for me, especially when he was 10 months because I was doing it with him in the Ergo carrier (couldnt carry the car seat and push a stroller).
Starting at age 2 he was big enough and we were just going to Magical Express so I rented him a CARES harness. We were all happier. He liked his car seat at 10 months on the plane. At 21 months not so much. With the CARES Harness, since his bottom was on the seat, I didnt worry about him kicking the seat in front of him. Actually he sat on a piece of cloth the harness owner recommended to keep him from sliding. What I really liked too was that he could sit in the middle, which meant I didnt have to. It also meant DD could sit by the window which made her happy. And of course my back was happy.