Well, if you want actual educational advice, here you go! This is going to sound complicated, but it really isn't that complicated when you break it down.
What you could do with her is try to get her to think metacognitively. You'll see tons of really complicated stuff on metacognition, but it basically goes like this: Planning, Monitoring, Reflection. It's dead simple.
1) She wants to draw. Ask her to sit down and decide what she wants to draw. Ask her what things she needs to draw what she wants to draw (pencil, paints, crayons, a reference image etc). This would be metacognitive planning i.e. she starts thinking about what she needs to succeed at what she wants to do. She doesn't start until she has everything she thinks she needs to succeed (also ups the chances of success). She might need help with this, so help her as her mentor.
2) Get her to start drawing. While she's drawing, get her to think about what she's struggling with. At 6, she's not necessarily going to be able to come up with ideas of how to improve her drawing by herself, but if she can identify the parts she finds hard, you can give her ideas on what to do to improve it. (For example: she's drawing a unicorn and having difficulty drawing the back legs - show her how you'd draw the back legs and allow her to trace it or copy it).
3) After she's finished drawing, heap on tons of praise on all the stuff she's done well, but also ask her first what she's most proud of in her drawing. Heap all the praise on that. You can then ask her what she found difficult and you can give her suggestions of what to do about that. Next time, when she's planning on drawing with you, bring those things up at the planning stage (eg. Remember last time we planned to use pencil but then you said you'd have found it easier in chalk? Let's try to use chalk this time etc.).
If she repeats this often enough, you'll 'train' her for self-improvement. It's the current 'in' educational theory, and the one that reportedly helps students improve 80% faster than any other techniques so perhaps it'll help? Just remember to keep it fun and simple and when she's drawing just for fun, let her draw just for fun.