Open iTunes and select the files you would like to turn into a mashup. Hold down the "CTRL" key on a PC, or the "Command" key on a Macintosh system, to select more than one song.
Navigate to the "File" menu without touching any keys on the keyboard. Select the menu, then select the "New Playlist from Selection" option in the drop-down menu.
Select the new playlist that was created and click on the first song in the playlist. Right-click the song title and select the "Get Info" option.
Find the "Options" tab and look for the "Start Time" and "Stop Time" editing options. Enter in the precise time you wish the song to start at by entering the minute first, separated by a colon, then the seconds, separated by another colon and the fraction of a second. Do the same thing for the stop time.
Select "Remember playback position" and "Part of a gapless album" in the "Options" tab. Then click "OK."
Edit each song in your playlist to play specific sections in the piece.