I've mentioned that key to improve is practice {practise(British English)}, then the question is how do/what kind of practice? This might helps:
**not #tutorial just explanation #how2do, may not suitable for everyone
**it applies on any kind of art(traditional & digital)
**learn basic drawing first is highly recommended before reading this

1. Takes your time slowly, if it's too hard you may trace it. The point of this method is to understand or knowing better your medium or tools you're using. For example: figure out how to use this tools to create shape like this and renders like that.
Even it takes several hours or days, keep push beyond your limit don't give up yet until it's as close as possible.

2. By doing quick sketch you will hone your sense(composition, point of view, color harmony,etc). Also some artist actually have their own 'templates' and workflow, try to find your own comfort zone.

3. Different than referencing only from photo, the 'information' from reality is unlimited such as smells, the atmosphere, sounds, reflection, everything you can capture and put it on your canvas.

4. I think if you've done those all i guarantee you can draw anything, if not, then use *beeep*
*jk

Conclusion: there are so many ways to improve, but the most importance thing is about "passion". If you love it, then you'll keep doing it.