Measure the length of the I-beam across the section of the basement where you want to frame it in using a measuring tape. Because the box that you will be building around the I-beam isn't load-bearing, you don't need to build it out of full-sized lumber. Making it out of 2-by-3 lumber and 2-by-2 furring strips will allow the box to take up less space and save you money. Cut four pieces of 2-by-3 to the measurement that you took using a circular saw.
Measure the height and width of the beam. These dimensions will be the inside dimensions of the box that you build. Cut enough pieces of 2-by-2 to these lengths so that you can use them as ribs on the box every 16 inches along the length of the I-beam.
Nail through the 2-by-3s and into the ends of the furring strips to create a U-shaped box. The 2-by-3s should run perpendicular to the U-shape, and the furring strips should make the U-shape, separated by 16 inches.
Hold the assembly in place around the I-beam. Screw it into the ceiling by driving 3-inch-long nails through the top 2-by-3s and into the underside of the ceiling joists.