Of course, there's more to it than just that, though that's a good 75% of the effort. On top of just writing, you have to focus on discovering the parts of writing where you need to improve, and then actually improving. So here's my three-step look at how to improve your writing skills.
To give you an example, my writing idols are J.K. Rowling, Suzanne Collins, Anthony Horowitz, and Hans Christian Andersen. J.K. Rowling to me is the goddess of children's fantasy, because she has a multitude of strong areas. The one that captures me the most, however, is her worldbuilding, which is a major weakness of mine. The world of Harry Potter is insanely real with incredible detail, the kind of world that you can step right into and lose yourself in, and she knows a plethora more about it that she hasn't even told us about. Suzanne Collins has a strong, relatable voice in The Hunger Games and the fact that she has created a main character who is strong and vulnerable, dangerous and virtuous, hard-hearted and kind, is an incredible thing. Anthony Horowitz I admire for his clarity and creative logic in his works, both his spy series, Alex Rider, where he demonstrates incredible logic and a knack for inventing spy gear, and his fantasy/horror series, Gatekeepers, which combines incredible concepts from literature into a clear progressive story. Hans Christian Andersen I partly idolize because we're similar in a lot of ways, but I also admire his skill with incorporating his own life and beliefs into his work, which is legendary now as common fairytales like the Princess and the Pea and the Little Mermaid.
See? With that brief analysis right there, I've already pinpointed six specific writing skills.
2) Figure out the skill areas in which you are gifted and the ones you need to improve on. For this to happen, you must have some kind of finished work to look at. Go over your writing, skill by skill, and see what it is you've done well and where your weaknesses are. This should be pretty clear, especially if you've got a few different pieces to work with. Peer critique is also helpful here.
3) Make improvements and expand your experience through writing challenges. This is actually one of my favorite things about writing. Not only do I get to express myself, tell stories, come to understand things I haven't before in life, and enjoy a whole new world, but I get to try totally new things. I set a challenge for myself with almost every novel. Generally it's simple or small, just about expanding experience, because the more you practice the better you get. A lot of my challenges have to do with point of view, for example.
For your understanding, here's a list of all the novels I've finished, whether I'm currently still working on them or not, and their chosen challenges. *Updated 5/24/15*
THE DRAGON SLAYERS -- first completed novel
THE ICE ENCHANTRESS'S PLOT -- first villain death
THE FOUR ELEMENTS -- writing from multiple points of view
THE GHOST CATCHERS -- first fantasy with historical basis, first character with a disability
THE LAST HOPE -- writing in first person point of view
FITNESS DAY AND THE END OF THE WORLD -- first contemporary novel
TWIST -- first story retelling
THE WALLS OF DREAMLAND -- portraying a romantic betrayal
THE PSYCHIC STORY -- writing partially from a male's point of view, first non-villain death
PERISHED -- switching up the characters' looks
THE CHOSEN FOUR Series -- first full series, first full book following a male (third person) point of
view, family inclusion
THE PROPHECY KEEPER -- writing a religious-based epic fantasy
WHAT IT TAKES TO DEAL -- writing a mixed genre and multimedia piece
The MERCIFUL Trilogy -- writing in present tense, first book entirely from a male first person point of
view, first book with a non-white point of view character
COCA -- writing from an unusual point of view