This is how in my personal preference (somewhat official. This is for a high power microscope.) First you take your slide and put it on the stage(specimen right over the hole). Then clip it on with the stage clips. Turn on the light, and look into the eyepiece. If you just see light , try adjusting the height of the specimen, or turn the focus knob.
Now for the student ones. Slide the slide onto the stage(once again, over the hole) and the stage clips should already be in place. Then adjust the mirror toward a light source. If the specimen isn't in focus, then twiddle the light focus knob(usually the eyepiece) and now it is clear.
A few words of advise for both of those. Always look with the body facing you. When you're done, make sure to put it where you found it. For the high power ones, in the bag, for the student ones back in the box.