Most car stereo head units are DIN and have a metal insert/slieve that fits into the dash opening for the radio (you bend little tabs on the slieve to make it clamp tight to the opening). There's also a back strap/support to keep the back of the head unit from bouncing around.
-Put the slieve in the hole for the radio.
-Install the back strap/support.
-Connect all the wiring to the head unit. (solder and shrink tube each connection; use the wiring harness adapter to connect the head unit to the honda wiring harness, assuming someone hasn't hacked the radio connection up.)
-Slide the head unit into the slieve.
-Secure the back strap/support to the back of the head unit and you're done. (the head unit is part of the dash and won't move.)