Hi there - there are a few ways to approach coloring/ outlining but, the main thing is knowing what your mediums do. So, in this circumstance moving forward, if you intend to use the same supplies, I would suggest you color first, let the markers fully dry then do the outline. What I do sometimes if the artwork is too complicated, after I erase my sketch, I gently/ lightly outline the artwork with a ballpoint pen. That way I have something to see if the coloring masks the pencils completely. Option B would be to outline the piece but, when coloring, don't go to the edge, give yourself "bleed" room but, I find option B to be a much trickier process. On the actual artwork itself... When doing outlines its always best to try and create smooth lines. This is best accomplished by putting down confident single strokes. What I do is I "practice" my stroke before I put it down. So, I hover over the line Im trying to create a few times so I get the distance, angle, and shape into muscle memory, then finally, I hold my breath while putting down the line. The dimensions/ 3-D - The proper way to accomplish this is to create a "vanishing point". A vanishing point is a point on your paper (or outside your paper) where all the lines eventually meet. So, if you took your ruler and extended the lines you currently have Id bet they never meet at the same point. Lastly, make sure all of the lines you create outward are the same length. Hope this helps. Keep up the good work!