Obtain Winamp, which you can download for free (see the Resource section below).
Download an MP3 output plug-in for Winamp. See the Resource section below for the popular Winamp MP3 Output Plugin program.
Install the MP3 output plug-in.
Open Winamp.
Select "Preferences" from the "Options" menu. You can also access Preferences by pressing on the "Ctrl" and "P" keys on your keyboard at the same time when you are in Winamp.
Select "Input" under the "Plug-Ins" category.
Select "Nullsoft MIDI Player."
Click on "Configure."
Select "DirectMusic / Microsoft Synthesizer (with output)" under the configuration options. This sends the MIDI data through the designated Winamp output.
Select "Output" under the "Plug-Ins" category.
Select "Chun-Yu's mp3 Writer." If you downloaded a different MP3 output plug-in, select that program.
Click on "Configure."
Select the destination folder for your MP3 file. You can also configure various other aspects of the output file, such as bit rate and tag format.
Close "Preferences."
Open your MIDI source file in Winamp. When Winamp plays the file, it writes a corresponding MP3 file to the designated directory.