How do you become a Technical Manager from an IC role?

As one who made this transition decades ago, first be sure you really want to do it not just because you want a promotion and more money. Once you make the move, your career will never be the same. In my experience, stepping back from management faced more resistance than moving into management ever did. And it’s a psychological adjustment going from a successful, well regarded IC to the new manager overnight. It’s suddenly no longer about what you know and can do (or have done). It’s about what your team knows and can do. Overtime, you will become less fluent in the details of a specific technology and have to rely upon the team to fill in the blanks. It’s trickier than perhaps it sounds. And for most us who made these moves, it took a while to reach a comparable level of job satisfaction. And some never do.

Start taking on the role of tech lead, where you are still an individual contributor but demonstrating project management capabilities of increasing complexity. For more on this topic, read The Manager's Path.

What do you mean "new companies don't want to make ICs into managers?" Where else do they get managers from? Do they only hire managers from outside? In the companies I've worked at, there has always been a lack of ICs keen to step up into manager positions.