Easiest - Spray ball on the end of a pressure washer
Then use antiformin rinse and peracetic . . . but as you probably don't have access to brewery chemicals, I would go with using a strong solution of a percarbonate cleaning solution (Whizz or Astonish - Which has no fragrances) and leave for 15 minutes . . . then drain, rinse a couple of times, then a litre of videne, shaken around to wet the insides . . . drain . . job done.
Fill the cask in the same way as you would a pressure barrel. Make sure you have a keystone in place in the tap hole, add primings to the pin, syphon the beer into the pin ensuring that the syphon tube reaches the bottom of the pin. When full add a hop plug, and hammer the keystone home . . . roll to disperse the finings and the hops .. .put away somewhere cool to condition. . . . a few of days before using you need to vent the cask with a hard peg, and bang it through the tut in the shive . . watch out for the fountain . . replace it with a soft peg to allow the excess condition to vent