I was told from my teacher... as i didn't do the prac either. I was sick... that you need to know the safety precautions of the experiment... so like knowing how the use the microscope safely, dealing with electricity and the blood slides. I was also told that you need to know that you estimated the size using a mini grid. Other than that NekkidSerpent was right.
Red Blood Cells = smaller, numerous amounts and no nucleus and approx. equal to the size 0.7um
White Blood Cells = large, nucleus, different types
Veins = proportionally thinner muscle