Many years ago I bought a kit from Photographer's Formulary. This kit include a small bag of metallic selenium powder. The instructions were to add this powder to water in a glass container, and then boil the water until the powder dissolved. The fine print said that the container should not subsequently be used for food, that the boiling should be done outdoors, and oh by the way, the selenium metal powder was fundamentally insoluble and so one should expect this process to take an extended period - perhaps hours.
Buying a bottle of KRS makes more sense.