It is said that there is a metaphysical repository of Reiki symbols, that can be viewed and studied. In addition, one's Guides, or the Reiki energies themselves, can selectively pass along one particular "symbol" to a person. The act of studying the repository for the purpose of bringing a new symbol down to "Earth" or receiving a symbol from Reiki or one's Guides is called "channeling" a symbol.
Generally speaking, one receives the symbol and writes it down. See more about Symbols as to what the written form, spoken form, name, etc. of a symbol really signify.
In Open Source Reiki, it is thought that anyone can channel a "new" symbol, write it, name it, etc. Other practitioners review and critique the symbols, use them, experiment with them, and expand upon their usefulness. Sometimes some are determined to be non-Reiki symbols (ask the Universe for an energy form, and it will not always be sourced in Reiki), and we've included some of those on the site as well.
How one channels differs from person to person. Some channel from within a crystal Reiki Grid, some channel while meditating, some channel during Reiki rituals, etc. There's no right way to channel.