Left-click first on the right-hand side of a standard star and record the pixel number. Then, left-click on the left-hand side of the same standard star and record the pixel number. Subtract these two numbers to get the diameter of the star in pixels. You should take two measurements for each standard star, one by you, D1, and one by your partner, D2, and then average the two diameters (D ave). Do this for each of the standard stars in the frame.