Open a log file that is the result of a TD-DFT calculation. For best results, a Gaussian TD-DFT calculation should include the following keyword "IOP(9/40=2)".

From the list of operations on the left, choose Electronic transitions.

Choose the start and end (in nm) of the convoluted spectrum, as well as the number of points you wish to have in the calculated spectrum. Sigma refers to the width of the band at 1/e height for the gaussian curves used to convolute the spectrum. The units for sigma are eV. (The equation is taken from Diedrich and Grimme, J. Phys. Chem. A, 2524, 107, 2003.)

After you have set the various parameters, click on the GaussSum logo to convolute the spectrum. The details are written to gausssum2.1/CDSpectrum.txt.