Purchase a swivel plate whose bolt holes line up with the bolt holes in your threaded flange.
Position a flange where you want the chair and mark the positions of the lower flange on the floor with a pencil.
Drill holes into the floor that are slightly narrower than the lag screws.
Screw the flange and pipe to the floor with lag screws, using a socket wrench. The diameter will depend on the size of the flange's holes. If the floor is cement, use masonry drill bits and screws.
Select a pipe that is 12 inches shorter than the height of the bar or table that you are using. Screw it into the flange on the floor, then screw a second flange onto the other end.
Separate the swivel plate's two disks.
Bolt one disk of the swivel plate to the upper flange of the pipe. The length of the bolts will depend on the hardware being used, but they should extend just far enough out of the flange to secure the nuts in place.
Use a jigsaw to cut a 14-inch circular seat out of plywood. Sand the entire surface, beginning with 100-grit sandpaper to remove any major defects, then moving on to 200 and 300-grit, to smooth the surface.
You can also use a store-bought stool seat -- if you do not want to make your own.
Paint or stain the seat if desired.
Screw the second swivel plate to the underside of the seat. It should be centered and attached with 3/4-inch wood screws.
Reconnect the swivel plate's disks.