With A Juicer
Prepare the oranges by first peeling them. Then, chop them into smaller parts to fit your juicer chute.
Feed a few into your juicer chute at a time- voila.

With A Blender
Peel the oranges and, optionally, remove and discard the seeds (if you're going to strain the juice then this isn't necessary).
Add the orange segments to a blender. You can add a little water to help them blend if needed (1/4 cup should be more than enough not to water it down too much).