Cut a circular piece of paper about the same size as your safety/shooting glasses lens and put it over one of your your shooting glasses lenses
It's also often recommended to use frosty Scotch tape for this--it blocks your vision but it doesn't block out much light to the eye. My understanding is that, when light is blocked to one eye, the other eye tends to compensate by dilating, further reducing the range over which the eye can focus. By using frosty tape, both pupils should remain as closed as possible for the ambient light level.
There isn't any need to do this at all, of course, if you determined you are not cross-dominant (right hand shooter, dominant left eye), or vice versa.