The best way to learn how to draw is to take someone elses work, and copy it, pixel by pixel, at fiorst.
Then, after you know all the lines, draw it free hand. Without copying, without having the picture there.
then change the picture. If it still looks good, change it again. Otherwise, go back a step.
Keep on goign until you ahve a picture nothing like the original, and you have a good idea about propotions and shading.
After those two, it's just learning to put lines where they should be, and which detials to include. if you chose a detailed picture to start with 9thanks, Dizi . . .

), you'll have a good idea about that, too.
problem with drawign on desks is that i always draw the same things. Draw other peoples work and your conscience keeps tellign you that your drawign is still too close to the orginal.