Before placing oil on the slide, focus on your sample as well as possible with the 40X objective. Make sure the light coming through the sample is optimal as well. If you are looking at bacteria, which may be very hard or impossible to see at 40X, you may find it useful to place a small grease pencil mark on the slide to focus on. Put the mark far enough away from your sample that it will not come into contact with the immersion oil as the oil will dissolve the mark and you'll have nasty black junk floating around your sample. When you have set the light and focus as well as possible using the 40X objective, rotate the lenses half-way to the 100X position. Place a drop of immersion oil on your sample and then rotate the 100X into position. You should be able to see the immersion oil making direct contact with the lens. Look through the eyepiece and perfect the focus using the fine focus ONLY.