Any help with how to get to the return for car rental.

I've done this twice and haven't any big tips except that I just followed the road signs and got there without a problem. You'll know when you are getting close (by the signage), so stop and fill up the gas tank then. I'm a planner and like to be prepared so was a little nervous about the car return, but found it was pretty stressless.

My recollection is that there is only 1 car rental return sign in the immediate vicinity of the terminal and that sign is obscured by a young tree. We had a devil of a time our first time.

Once we located the drop-off, however, it couldn't have been easier. Basically, a big multi-storey parking garage right by the terminal.

(There is a gas station at the entrance to the airport property, too.)

I presume there is an airport website that gives a plan of Fiumicino. Print that and bring it with you.

This is from ~7yrs ago;

Over the years I believe some people have had a problem because they missed the 1st large billboard sign ~1mile or so before the airport.

That sign tells you what circled letter to follow.
This is just an example and not accurate.

Let's say there are 5 letters A-E, each letter is in a different colored circle.

(A)is for Terminal A, (B) Terminal B, (C) Terminal C, (D) for the Parking Lot & Rental Car returns and (E) for Cargo & deliveries.

Ok so you want (D), you come-up to a split in the road A-B-C signage points to the right and D-E points left, go Left, the next split is D-Left & E-Right, so another Left, now look for the sign for Car Rental Returns.

Now heed tedgale's warning, 7yrs ago the tree was no problem for me but I recall it suddenly appeared and the lettering was rather small and called for a quick Left?, perhaps??? even just before a traffic merge so the driver would be occupied looking over his left shoulder just before the sign.
So having a co-pilot just looking for the signs is a big help . Regards, Walter

It's relatively easy, but heed the comments by tedgale and paradiselost: we were there last year, and had no trouble seeing the sign telling us which letter to follow. However, the sign pointing the way to B was pretty small and low. Thankfully, we saw it and were in the correct lane. Since it's one-way, I imagine we would have looped all the way around the whole airport to get another chance.