You must have capital - you have to be able to get up and running and sustain your network at a level that is necessary to keep your audience interested, the advertisers happy, and the cable operators desiring to keep you on board. So knowing you are a viable business begins the process.
Here you would need to create a business plan and lay out your entire idea and how you plan to spend and make your money. I would also create a programming schedule that is 24/7 for one week, with highlights for a month and how you plan to spend your year. I don't know what your idea is, but if it copies something that already exists, your changes for success are not very high.
When you suggest buying out other networks, I am not sure what you mean - those networks may not want to be bought out - and may have long term contracts with cable companies. It is a very competitive business and all you need to do is dial around and see that you have a lot of competition.
Having said that, if your idea is unique, you think it may be attractive to a large national audience, you have the business plan, the capital, and the legal to continue, you may be onto something. Then you can contact the homebase for the large cable/satellite operators and see if they have any interest in your network.
But like any business you need to study your competition...what is their schedule like? How might they be producing their shows? What is their potential budget for operations? Where do they operate their programming from? Does your programming match up to what see currently existing?
The major media corporations like to keep it in the family. That is why you don't see numbers on how to do this or any kind of assist as to how to start up this business. So you are left to speculate and just take a stab at it... Any contacts you can make in the business will help, but it does make for a very tough animal to corral.
This is a cash intensive business - but if you work at it, have a sensational idea, and can pull it off - you may just make it. Best of luck on your venture!