Its truely not necessary to have the farrier do it however if you have a sore horse who needs support NOW.
Although after a fresh trim is great, they also make a great temporary shoe if necessary if a horse throws a shoe or so forth. They don't need to be padded (in fact I prefer no pads) but you do need to cure them with the foot loaded (IE pick up the offside foot with the casted foot on a garden kneeling pad so it conforms to the foot.
JMO as a trimmer.
Heres a video.
Casting is available in do it yourself kits from several places if you wish to purchase them that way. I buy it by the box from medical supply places. Lots cheaper.