I have not used a plastic bag. I make my own bags out of spinnaker cloth, which is just as protective as a plastic bag. I turn the handlebars, remove the pedals, lower the seat, let the air out of the tyres if they insist, even though there is no need. I don't remove the derailler, but some do.
If you are going to bag your bike up at the airport you will need an allen key for the handlebars and the correct spanner for the pedals. HINT>>>> make sure you can get the pedals off before you leave home. I have yet to find an airport who can supply any tools to help. I saw a guy searching for a large stone at Majorca airport to use as a hammer. You don't find many large stones at Majorca airport, apparently. Also check if you have to let the air out of the tyres before you bag it as you cannot get to them once the bag is taped up and you will have to undo the whole lot again.
I have flown lots of times in Europe with bikes and have never had to box it and a couple of times have not needed to bag it either. You will find that airlines do not have a common policy when it comes to bikes. In fact you may also find that the same airline has different policies at different airports. That's not strictly true. Some staff do not know their own policies.
I had a bit of a confrontation at Begamo airport where Ryanair was not going to let me on the plane unless my bike was boxed. I had printed out their own information which at the time said it needed to be in a bag or box. I also asked the guy in charge how he thought I got the bike from Denmark to Bergamo in the first place.
I have dropped into the Ryanair office at Billund in Denmark to ask what a "Protective bike bag" is. They had no idea but "guessed" it was a bag that the bike went into. That makes my bike bags and the plastic bags ok then.
I have never had real problems flying my bikes, just a couple of niggles.
The important thing is to read their info and print it out to take with you. They do not define what a Protective bag is. My sail cloth has UV protection, that will do. If its in a secure bag it will be fine as long as you smile a lot and say Good morning to all those who think they have power over you.
But the best bit is when you get to the other end and you put your bike together, stow your bag and pedal off into a new country. It makes no difference if you are pedalling in the wrong direction, because you will. But it is so satisfying passing all those riders waiting for taxis and dragging bike boxes on wheels.