looking good so far!! I really like your line work! It has a really nice flow to it 
If you're looking to further refine your sketch work, there's unfortunately/fortunately a gazillion different ways to go about it when it comes to actual refinement, but the best advice i can offer you is to set yourself up for success!
Most of the context for this i'm referencing is for digital art, but i'm sure you can find ways to apply it to traditional art too! The key part is leaving room for your to be further developed, should you want to. This doesn't mean leaving things unfinished, but finishing the whole first step, so you could easily move to the next if you need! This manifests differently for everyone, but i've noticed a trend of either line art or values in places for the sketch (if that makes any sense lmao)
An artist I follow actually goes pretty in depth about his process if you wanna check it out! He worked for Riot Games for a while, and is now working at Wavedash games! I'll link 'em here --
Refining Sketches
Finishing What You Start
Hopefully that's helpful!! Your work is wonderful and I see a lot of potential!! Keep it up!! 
