Examine the area you will be filming/photographing and determine the area where the camera will stand in relation to the subject. For the most accurate manual focus, this information needs to be decided prior to the focus being set. Walk around your subject in order to determine the best angle. If your subject is mobile, move the subject around an area until you decide on a framing and composition that you enjoy.
Use a tape measure to find the distance between the lens of the camera and the subject. Depending on the lens and how far away the subject actually is, you may need to find this measurement in millimeters or feet.
Rotate the focus ring on your camera lens until the marker on the ring matches the distance you measured in "Step 2." For example, if the distance between your camera and your subject was measured at 100 mm, rotate the focus ring until the marker is pointing toward the "100 mm" marking. Your manual focus has now been set and you can photograph your subject.