There are a number of methods to do this, one of the best is to use a stereoscope where each eye looks through a lens at its own image. However stereoscopes are not always readily available. Fortunately it is not too hard to view stereo images with no equipment. A stereo image may be presented in one of two formats. The first is with the right and left views displayed on the right and left of the image pair. The technique of viewing this type of stereo image without extra equipment is called wide-eye, wall-eyed, or relaxed-eyed viewing. The left and right views may be displayed with the two views switched left and right. Viewing this reversed format is called cross-eyed since the eyes are cross a bit so that the right eye views the left image and the left eye views the right. Cross-eyed viewing is actually a bit easier to master.
Here is a test image for practice. Cross your eyes while looking at this image. The two sides will appear to split into two overlapping parts for a total of 4 circles. As your eyes are crossed more the split images will spread apart with the two center ones moving toward each other. Bring the two center images together into one. You may have to tilt your head slightly to keep the images aligned vertically (keeping the images aligned vertically reduces the strain on the eyes). Do not try to view the images from closeup, that actually makes cross-eyed viewing harder. A useful trick is to hold your finger about half way between your eyes and the screen, just below the image. Focus on your finger and move it back or forth until the center two images merge into one. Then slowly let your eyes focus on the image beyond your finger. This technique may seem hard at first but with a bit of practice it becomes very easy. The stereo images available elsewhere make the effort worthwhile.
When you get this in cross-eyed stereo you will see the left small square floating in front of the circle and the right small square behind. If this is reversed you are doing wide-eyed stereo.