Cost: about 3500 yen
Duration: about 2.5 hours
1. Take the JR Narita Express ("NEX") to Shinjuku. This takes about 80 minutes, and is quite comfortable, with announcements in English and refreshments and toilets available on the train. The one way fare is about 3200 yen. When you buy a ticket at the station ticket office you will be given a specific seat number. This is the fast and easy part; after arriving at Shinjuku the instructions are the same whichever way you choose to go.
(It may be convenient to meet your Japanese host here, on the Narita Express platform. If you telephone your host from the airport, giving the departure time of your train, he or she will be able to meet you here at Shinjuku and accompany you to the university.)
If you need a receipt for the Narita Express fare, ask for one when you buy the ticket. Afterwards it will be too late, as your ticket will be swallowed by the ticket gate when you leave JR territory at Shinjuku. Alternatively you can tell a ticket collector at Shinjuku that you wish to keep your ticket, and he will return it to you after stamping it.
2. From the JR Shinjuku station, go to the "Keio Line" (part of Shinjuku station operated by the Keio Company). Shinjuku station is rather large, so you may have to ask somebody where the Keio Line ("Keio Sen") is. The easiest way to find it is as follows: when you get off the Narita Express, you will be in the JR station. Look for the steps leading down from the platform, and follow the (small) signs which say "Keio Line" in English (the signs are visible as you are walking down the steps). It is possible to go through a ticket barrier directly from the JR station into the Keio station, if you already possess a Keio ticket. Since you will not have a Keio ticket, however, you should go through the adjacent "exit" ticket barrier and look for the Keio ticket machines. These have "Keio" written in English.
Ticket machines are very similar for all railway companies throughout Japan. Above the machines you will see a map from which you can determine your fare. The price of a ticket from Shinjuku to Minami-Ohsawa is 390 yen. Put enough money in the machine, until the amount (i.e. 390) is illuminated. Then press the button, and your ticket (and any change) will come out.
If you are not confident about which buttons to press, just buy any cheap ticket (eg 200 yen) at Shinjuku, and pay the rest at the exit ticket barrier in Minami-Ohsawa. This is legal (and very convenient). You must show your ticket when entering and leaving the station, but it will probably not be checked on the train itself.
If you have luggage, take any train for Hashimoto. Trains leave every 20 minutes. Journey time to Minami-Ohsawa, which is two stations before Hashimoto, is about 40 minutes. You can arrive a few minutes earlier by taking any special express or semi -special express (to anywhere) and changing after 15 minutes at Chofu.