The monastery is up a long hill overlooking the valley. If you are coming from the airport, take a taxi from there to Kopan, which is near Boudhanath. Ask for Kopan Gompa. The taxi from the airport is approximately 600 rupees if paid in advance at the Fixed Cost station in front of the entrance. You can get to Kopan for less through negotiation, but this price will get you there without minimal hassle.
Drivers accept tips.
If by bus, go to Boudhanath. Then you will have to walk approximately 40 minutes to Kopan. The last half of the walk is up a long paved road on Kopan Hill . Before the paved road the roads are often muddy, especially in the summer. There are no signs, but you may be able to get directions by asking for Kopan Gompa.