When I was 16, I was still unable to dive correctly. I knew what the problems were, but I could not correct them, and I didn't have a coach. So I devised my own Shadow Goalkeeping method. In my imagination I would replicate a series of shots on my goal and respond physically. Without question, it helped me develop my side diving technique and recovery.
Imagery is one of the tools of skill learning today.
Modified version with simple saves. Try the Mirror Man routine below. 12-15 Goalkeepers on a regular basis. Occasionally have the whole team go through the routine. The same principle can be used with the field players, and they don't have to be soccer actions.