Say you've got a large room that you want to break up by putting in a raised floor at one end. Even though it's only going to be raised by one step, you still have to set up a floor joist system and top it with subflooring. It's an involved project that you shouldn't undertake unless you have strong carpentry skills. This plan assumes the new raised floor will be open to the lower part of the room on one side, as it really wouldn't make sense to do it in a closed room with just a doorway leading in. It also assumes the room has been stripped of all floor coverings, trim and other obstructions, and that the floor you're building on top of is level.
Go around the room with your stud finder and your roll of masking tape. Put a strip of tape on the wall to mark each stud.
Starting at the inside corner of the room, lay your first piece of 2 x 8 next to the wall, lying on its face. Spread carpenter's glue along the back of it. Stand it up on its long edge and press it to the wall, with the edge sitting on the floor. Screw it into the wall using your screwgun and 3-inch wood screws. Place three screws along the width of the board into each wall stud according to the tape you've placed on the walls.
Continue around the room, putting 2 x 8s all along the perimeter. Measure and cut the boards with your miter saw as needed to make them fit. For corners, let one piece go all the way to the wall, then butt the perpendicular piece up to it.
For the open end of the room, where there is no wall to run it along, screw it into the edges of the perpendicular boards at each side. Screw a second board to the inside of the first one, doubling it up to give it added strength.
Put your tape measure at the end one of the long walls. Measure 16 inches and mark it on the 2 x 8 that's attached to the wall. With your square, make a second mark 3/4 of an inch back toward the wall from the first mark. Put an X where the first mark was. Measure 16 inches from the second mark, and mark another line, and then put another X outside the line, on the same side of the line as the first X was. Continue this all down the wall, and on the opposite wall. You'll end up with lines every 16 inches, with X's on the same sides of the line to remind you of which side of the line the joists should go.
Measure the width of the room between your two anchor pieces and cut your first joist at that measurement, to span the width of the room. Wedge the joist in between the two anchor pieces so one side of the joist lines up with the 16-inch line and the width of the board is at the X. Use your screwgun and 3-inch wood screws to attach the joist to the anchor piece, putting the screws in at an angle on the sides of the joist, two to a side.
If your joists are more than 10 feet long, put staggered braces at the centers. Each brace should be 14-1/2 inches long (to account for the 16 inch space between the joists, minus the 1-1/2 inch thickness of the wood). Secure them by sinking 3-inch screws from the other side of the joist. Stagger them enough to get to the back of each brace from the other side of the joist.
Once all the joists are in, lay the plywood tongue-and-groove subflooring over it, with the length of the plywood running in the opposite direction as the length of the joists. Spread carpenter's glue over the top edges of the joists as you lay each plywood piece.
Secure the plywood with your screwgun and 1-1/2-inch screws. Make sure the seams of the plywood that run in the same direction as the joists are resting on the tops of the joists. (The plywood sheets will be 96 inches long, which means they will always come to the centers of joists positioned at 16 inches on center.) The raised floor is now ready for any kind of floor covering you want to put on it.