Double click your iTunes icon on the desktop to launch your program. Click the "File" button on the top, then click "New Playlist."
Name your playlist, then drag your music to the newly created playlist. Drag your music files around to manually set the playing order. Click the "shuffle" button on the lower left of your program to have iTunes automatically select the order of music.
Click the "Edit" button at the top to remove downtime in between tracks and mix your music. Select the "Preferences" tab.
Begin to mix tracks by selecting the "Playback" tab and checking the "Cross-fade Songs" check box. Increase or decrease time for songs to fadeout before the new one begins. Moving the fade button farther to right increases the overlap time so that before one song is over the other begins, removing any downtime pauses between songs.
Click "OK" to save your changes, then double click your playlist to begin playing your iTunes mix.