Leeuwarden is placed in the province of friesland in the north of Holland. It is far from any airport.
There are two possible ways to get to leeuwarden. The first one is via Amsterdam city. In Amsterdam is possible to take a trains from the airport directly to Leeuwarden. The disadvantage is that the flights use to be more expensive than the next way.
The second way is via Eindhoven placed at the south of Holland. It is farther than Amsterdam but the flights are cheapest. The disadvantage of this way is that you have to do some changes before you get to Leeuwarden. First, from the airport it is necessary to take a bus whitch gives you to the train station. From there You have to take a train to Utrecht city in where another change is necessary. Then at the end you can take a train directo to Leeuwarden. This way is pretty much longer than the first.
The train schedule and the tickets are avaible at www.ns.nl
There is also a great group discount so it is better to travel in group.