I have 2003 Honda Civic LX Coupe 5 speed manual trans where do I put in the fluid

Hi, what year is your civic lx coupe? Until 2005 the Civic manual transmission fill plug is located on the right side of the transmission above the drain plug. The drain plug will be round with an internal square 3/8 hole for removal, and the fill plug is a 17mm bolt, like your engine oil drain plug, located about 4 inches above it and to the rear. It will look a little like the attached image. Simply remove the fill plug, use a funnel from above the hole with a hose attached to it to fill the trans with fluid. You know it's full when it starts running out, so fill slowly.