(dal sito ufficiale Celestion.)
Break in a speaker with a fat, clean tone: turn up the power amp volume to full, and control the level with the preamp gain. Use a level that will be quite loud, but not painful in a normal size room.
Have the bass and mid up full, and the treble at least half. On your guitar, use the middle pick up position (if your guitar has more than one pick up) and play for up to an hour, using lots of open chords, and chunky percussive playing.
This will get the cone moving, and should excite all the cone modes and get everything to settle in nicely. The speaker will continue to mature over the years, but this will get it 95% of the way there.