We used to steam hay for one of our horses are she had COPD. All you do is put the hay in to a plastic container that has a lid, we used to use the big plastic bins as we were steaming about a bale every night for her. Boil the kettle and pour it in, we used too have to put around 5 kettles in but if your only doing a slice then you put a lot less in. Then close the lid tight and leave it till it has cooled. It smells really nice when its steaming we used to warm our hand up on the bin in the winter when it was freezing lol
We used to feed the horse it when it had cooled but for the pigs you may want to do it the night before and let it dry.