How do I become great at structural steel design?

Start by making steel drawings (plans and details!)  so you see a lot of different structures, then start with calculations and detailing steel structures, so you know where the problems are. But don't forget to look at your work on site and talk to the people that prefabricated the steel and now put it together, listen to them as well.

you should know international codes for steel like  LRFD , BS5950 and know AISC , you should have experience in use Staad Pro and pro steel for design and analysis & Tekla structures and StruCad for detailing of steel structures. 

Improve your ability to understand & draft connections, then connections then connections. (both welded & bolted)

You have to love this domain and to be up to date about all new things about it .
you have to study primaries about steel design ( beams - columns , connections) how does it work to be sensitive about it .and you have to know that the connection is the most important study.
you have to work hard and able to use the structural steel design programs (robot - sab ...
etc) More importantly, all of this can be to have the desire to achieve this goal.
And always looking for innovative solutions in the design which give safety design with the lowest cost