I would say buy this book for starters:
https://www.amazon.com/Never-Eat-Alone-Secrets-Relationship/dp/0385512058 It is absolutely fantastic for learning how to expand your network. Many of the lessons in the book are guiding how I am going about creating contacts at every IB prior to B-school.
Then add:
1. Chat up everyone you have time to have a 3-5 minute meaningful conversation with at networking events, but don't just wisp around the room talking to everyone for 30 seconds. The more people you meet, the more people you have going to bat for you. Just try and have as many
meaningful conversations as possible. Also, say hi and have conversation with the people you know and are comfortable with at your target firm, but try and branch out as much as possible and cast your net wide.
2. ALWAYS Follow up - especially important, it is the only way to keep a relationship going