How do we practice Wing Tsun or any martial art solo?
There have been many periods of time that I have had this as my only option. I used to go through periods of being frustrated that I didn't have an accessible teacher, much less training partner. Early on, I got some sagacious advice that I have found to be true in my own experience.
When you are without a training partner, first you practice the forms. When you can do those well, you work on coordination. That is your feet and body are balanced and you are able to coordinate the movement of your hands and feet in slow methodical training. For me this was an important plateau to cross. Then, once you can do that all you have left is shadow boxing.
A few times a week, prior to having a local group to teach, I would train these exact things in my garage in front of a mirror. Its hard to judge one's progress like this but each time I was able to get back with an instructor I found that despite being apart for a prolonged period of time (weeks to months) I hadn't built up rust. On top of that, I built some fluidity and crispness to the things I was practicing. So I ended up arriving to train needing little review of things that had already been covered and could see new material.
Can you get good this way? Definitely. Improvement in muscle memory, coordination and fitness are always broadly useful. That said, I would practice the same error many times before being told it was an error and though I could refine the skills I had, it was more a sharpening than a growing per say.
When I get the chance to train with an instructor I also get as much information retained as I can. I tend to get large boluses of information in short periods of time (hours or days) which is much more than I can retain. So after each training session I sit down with my journal and write down everything we did, specific comments made regarding my technique and my own internal thoughts. I then review these notes on my way to train with my instructors again. In this manner I am able to retain 80-90% of what we trained months or weeks ago.