For each display you want to use, connect a video cable (and
adapter, if necessary) from a video output port on your Mac or graphics
card to the video input port on the display. Make sure the cables are
securely connected or you may not be able to see an image on the
display.
If your Mac has one or more Thunderbolt ports, you can connect
up to two displays to those ports. If the displays also have Thunderbolt
ports, you can connect one display to another, and then connect one of
the displays to a Thunderbolt port on your Mac. Or you can connect each
display to different Thunderbolt ports on your Mac.
If a display is not recognized, click the Detect Displays button
in Displays preferences, or try putting your Mac to sleep and then
waking it.