Measure the total space assigned to the clothes rack, in order to know what length your gas pipes need to be. Decide on a convenient height. Typically 36 inches for the height is a good starting point, unless the rack will lean against a wall that already has shelving, in which case the height should be adjusted to fit well beneath the shelves.
Buy the gas pipes at a hardware store and have an employee cut the pipes to the measurements you decided upon when measuring the space. Buy eight three-way connecting gas pipe corners, needed to assemble the rack. Make sure the ends of the pipes are not rigged, since they will be glued in place with caulk.
Spread out the cut gas pipes in the space where the rack will stand. Choose a three-way 90-degree connector, and fill all three openings with caulk. To create the bottom left corner of the rack, fit a pipe cut to desired height measurement to the end of the connector going up. Repeat this process for the pipe cut to the desired depth measurement to the end of the connector going straight forward. Repeat once more for the pipe cut to the desired length measurement to the end of the connector going right.
Insert all pipes into their connectors using the bottom left corner created as a guide, until a rectangular cube shape is created, forming the clothing rack.
Let the caulk dry fully before hanging anything on the rack, and use a craft knife to cut away any excess caulk that may have come out of the openings when inserting the gas pipes in place.