Determine what is causing the hot room. If the room has only vents that blow air then you will need to add a return/bypass duct. If you have two vents and one blows air and the other sucks air then add a supply duct. Most older houses do not have air conditioning return ducts in each room.
Make a material list. All air conditioning flex connections will need duct tape, straps, and mastic. To add a supply duct you will need a 10X6 boot, two 6" flex collars, 6" flex, 10X6 supply grill. To add a return duct you will need two 10X10 boots, two 10" flex collars, 10" flex and a return grill.
Buy all the materials you need before you start. A home improvement store will have everything you need. Oddly the parts are often in the plumbing department.
Position the air conditioning boot in the attic above the hot room. If it is a return duct that you are adding then install it near the opposite wall from the existing supply. A supply duct should be positioned to blow into the hot area.
Cut a hole in the hot rooms ceiling from the attic down into the room. For a return cut the hole 10 1/4 X 10 1/4. For a supply cut the hole 10 1/4 X 6 1/4. The hole should be 12" off the wall and 2" from a truss. The hole needs to be cut from the attic so you know where the trusses are. Move the boot to another wall if needed. A return will need another boot installed as close to the air conditioning unit as possible. Sometimes there is a large one already in the ceiling, if so you can just tap into that.
Hold the boot in the hole while a helper attaches the grill from below. This will hold the boot in place for now. Screw boot clips or angle to both the boot and trusses.
Stretch the flex from the vent to either a supply mixing box or to the other return vent. Place a collar on the vent and trace the hole. Cut a hole with a knife. Do the same on both ends of the flex. Install the collars. Make sure to bend the collar tabs over.
Hook the flex to the vent collar. First push the inner liner over the collar. Use duct tape to secure it in place. Tighten a strap over the inner layer of flex. Pull the outer layer as well as the insulation to the vent. Do not leave a gap. Tighten a strap over the outer layer. Seal the outer layer to the vent with mastic.
Cut the flex if it is to long. Use a knife to cut both the inner and outer layers. Use wire cutters to cut the wire.
Hook up the other end of flex the same way as above. Use straps to tie the flex to trusses.