Fitting Aerobars
You might be able to get away with just slapping them on and moving forward on your saddle so your elbows are 90 degrees.(I know its not very 'correct' or professional, but that's what we do on my cycling team for TTTs) If you find you are too stretched out and can't pedal at full power, then a kind of solution I know some people have is to have a shorter stem to use when they are using their aerobars. Get on a trainer and play around with it.
I'd suggest going back to who fitted you originally. As seat position angle of seat and the areobars are all affected. Once they have you dialed in pretty close - mark all of the positions as a baseline and then if necessary you can tweak it a bit. As things will feel different after a long ride versus a 1 - 2 hour ride. And when making adjustments never change more than one thing at a time. And usually no more than a mm or two. Then take it out for a good ride and repeat as necessary.
RV
It takes a long time to get good. - Scott Molina
Slow is smooth; smooth is fast. - Rich Strauss

I've just picked up a set of aero bars for my road bike. How necessary is it to get them professionally fitted or are there instructions somewhere on how to do this yourself? I don't want to go messing up my bike setup which was done professionally if there is a risk of doing this. Also, I doubt I'll use the aerobars 100% of my riding time, I'd imagine only on long straights, so if you need to change your set up for them won't this mess up your set up for the rest of the time??? A bit confused as you can tell. :confused: