How

First, it is necessary to study and to bear in mind the geographical disposition of different countries and cities on the map of Europe and based on the cities where you would like to go do a first approach to develop more convenient and efficient route.The route should be drawn keeping in mind the supplements or reservations required for some connections. This information can be found on the web (the website of the German trains - www.db.de - is even used by employees of railway companies from other countries), but many people prefer to buy the "Thomas Cook European Timetable Railways, this book can give information on almost all routes of railways in Europe: time, distance, type of seats available, reservations required, supplement, etc.. which can be of great relevance both before and during the trip ... In my opinion, although it is a very useful tool I think it can be optional.The cities that must be well studied are the first and last. It is natural in order to capitalize on the inter-rail to the fullest the returns would be on the last day. Therefore the last trip can not fail since if for some reason if you get trapped in some of the scales you might get the risk of having to pay the rest of the trip separately. The first city should also be thoroughly studied to avoid delays on arrival.You can also try to balance the time that you will stay in more expensive cities (typically in Western European countries) with the cheapest (Eastern countries), the longer you spend in these last countries the less will be spent.

One should never think that the trip will always be according to plan. On a 30 days trip inevitable and unforeseen obstacles will happen along the way and of course you will be required to changes your plans, e.g. cities for which you plan to stay 3 days you might stay 1day, or a train stopped to make a connection, or a delay, etc..

Finally during the preparation of the Interrail you should also study some cities where you are planning to visit. Making a list of points of interest, addresses of hostels or other accommodation, etc... How? Use the Internet!