For general tools which support a GNU-like --help flag, you can use the _gnu_generic, like:

compdef _gnu_generic my_command ls youtube-dl

Sometimes if the --help description is multiple lines, it doesn’t include the entire description or may break.

There’s a similar command in bash; complete -F _longopt my_command

This Tutorial, and the corresponding zsh-completions is great for learning how to do zsh completion.

Examples

For basic completion which doesn’t need state, to autocomplete a command as the first argument, and then some filename as the second argument:

# for something named 'command'; if your command was 'hello', the function would be '_hello'
# you can always manually invoke a function to complete something with compdef as well:
# compdef _command command
function _command() {
    # typically this would be done with the built-in _path_files
    local files="$(command ls)"
    # 'files' and 'command' are just descriptions
    _arguments \
        '1:command:(create remove)' \
        "2:files:(${files})"
}

If you want to have 2 commands and complete multiple file arguments, you can use a glob:

function _command() {
    local files="$(command ls)"
    _arguments \
        '1:command:(create remove)' \
        '2:command:(force quiet)' \
        "*:files:(${files})"
}

e.g. command create force file_one file_two

_arguments is just one of the possible utility functions, there are a couple others.