I'm landing on Iberia in Fiumicino, Terminal B... I plan on taking the Leonardo Express, but my Italian is very limited. Are there signs that make it clear and easy for someone with limited Italian to find the Leonardo Express from inside the terminal ?

Yes, it is easy. There is a train icon o the sign.
Further down on this page are directions on getting from baggage clain to the FCO train station.
http://www.slowtrav.com/italy/planning/rome_airport.htm
You can buy the tickets out of the automated ticket dispenser (English is one ofthe options) or you canbuy a ticket for Termini (the Leonardo) at the newpaper stand at the train station.