First of all, you may not realise it but you already belong to many networks: work colleagues (current and former), old school mates, church groups, friends, etc. How do you take advantage?
First thing is to identify who might be useful to you in your career and target them. Successful people tend to have large egos. Massage it. Be humble. Don’t be too proud to tell them you admire their work, but don’t act like a star struck teenager. You’ll look pathetic. Don’t.
Note that you often have to give before you can receive. Everybody likes gifts, no matter how rich they are. Don’t give in expectation though. You’ll look cheap and desperate. Giving doesn’t have to be gifts. It could be offering to help the person to do something they need done.
Let people know what you do and what you are good at. This includes your old school mates, friends, elders, politician friends, business colleagues, etc. Write often in appropriate journals, magazines, newspapers, etc. Let people know you are the go-to person on that topic.
When you can afford it, occasionally organise a drink up at a nice venue. Invite top people who are likely to respect each other and just let them bond. Show them respect. No need to tell one person the other is coming. Make each person feel they are the most important invitee.
Let them just jam there “accidentally” and express “surprise” that they know each other. One top person will feel that the fact that you know the other top person must mean that you are alright. Be friends to everyone. Don’t yab people behind their backs. Don’t burn bridges.
Ensure that people can trust you with secrets. Many top people will tell you some very personal things and then wait to see if they’ll hear it elsewhere. Many people will boast they know those top people in a beer parlour and say things you know are untrue. Don’t argue with them.
Just plant a doubt in their minds with a question and then leave. Remember that out of sight is REALLY out of mind o. There are many people competing for attention. Every so often, do something that reminds people that you exist.
Check on your senior friends every so often. They really appreciate it. Top people are always surrounded by people but are actually very lonely. Asking how they are, how their health is, how they are coping with work, etc, can be very touching for them. People often don’t ask.
Finally, respect yourself and don’t be a burden or a nuisance. Pay when you are supposed to pay. Be very selective in who to include in your network but be open to important new people that may help your image. Focus on the friendship and not on its potential benefits.