Measure the sofa to make sure the 30-inch tall and 46-inch long table will fit behind it properly. If not, adjust the size of the pine boards accordingly.
Sand the edges and top of the 46-by-18 pine board until nicely smooth. Wipe any dust away with damp cloth.
Measure the curves of the sofa legs and set the lathe to those measurements. Mark on the table legs a 2-inch space at the top of the leg boards. Set the lathe based on the curve measurements of the sofa legs. Begin turning the legs on the lathe under this 2-inch space.
Sand the legs until perfectly smooth. Use wood screws to drill an L-bracket flush with the top of the legs on the two sides that belong on the inside of the table. Using these L-brackets, attach the legs to the bottom of the tabletop in the corners so that the top of the leg is flush with the edges of the tabletop.
Stain the table to the same shade and depth of color found on the sofa legs using the staining brush.