find information about the existing specifications in this family;
find the code points that have already been registered;
request to register a code point;
find information about reference software for the file format.
Look at the existing specifications and practice; can your need be met with an existing definition (possibly from another organization)?
Can you use an existing extension mechanism (e.g. a UUID box)?
Is what you are defining stable, defined in such a way as to have longevity, and flexible enough to be a good general-purpose, public, design?