Work on your isometric boxes so you can draw the construction box better which can help you in sketching!
Try to draw the isometric boxes properly then you would be able to have a good drawing (:
Your isometric boxes are not properly drawn , try to improve on them.
Improve on your isometric boxes, try to draw it more straight! ok? =)
You should practice more on your isometric boxes, also, your parallel lines need a bit of work here and there, overall, good effort!
Your lines need to be straighter,but still,good job!
You need straighter lines and better proportioning.