First of all, i would get some blank white paper. Lines or squares tend to deform drawings and can dictate proportions that are not necessarily good (I do not want to say "right" here, because everybody has a slightly different view on proportions). Concerning proportions, I found it helpful to copy photographs or ideally real models, but the most important trick is practice. Loads of it. As much as you can, although it will always be way to little.