Start with Sodium Percarbonate, seal the keg, shake to get pressure up then vent the connects to blow cleaner through the pipes and connection pillars. Shake every 5 minutes for around 30 minutes, vent connects again and empty.
Rinse with cold water, then add boiling water and seal. Shake it up again and vent steam through the connects.
Finally, star san and a touch of pressure and blow that through the connects.