Ian,
This depends on how many changes you have made. You may have to do some manual editing to get your changes into the child theme so that the parent can be updated without affecting your changes.
Start by creating your child theme normally. Do not activate it yet. Make a backup of your parent theme (download it via ftp or copy using your web hosting control panel).
If you have made changes to any of the parent theme template files, copy them to the child theme using the Files tab. Do not copy all the templates, only the ones you have modified.
If you have made changes to the parent theme functions.php file, you will have to copy them to the child functions.php file as “pluggable” functions (keep the function name) or as new functions (use a unique prefix like ‘child_function_original_function_name.’ The former is preferred as it requires less code and doesn’t require changes to the actions/filters.
If you have modified the parent stylesheet it gets trickier. First you have to MANUALLY copy (offline) any and all of the edits to the child theme style.css file.
If your theme uses additional stylesheets besides style.css and you have edited them, add these edits to the SAME child style.css file.
It may be helpful to use a text editor that allows you to compare differences (TextWrangler or Notepad++) and diff the unmodified theme stylesheet with your edited version. Then move the differences to the child theme stylesheet.
Once you are comfortable you have copied all the edited styles to the child theme stylesheet then run the configurator generation (under Parent/Child tab) again. This will parse both the parent and the new child stylesheets into the configuration data.
Test your child theme using the Live Preview under Appearance > Themes to make sure it looks right. This will also tell you if your edits have any syntax errors.
The next version of CTC includes an option to copy parent theme options but for now You may need to set the menus, widgets and other theme options for the child theme using the Live Preview as well.
Once everything tests correctly, you can activate the child theme. At this point you should replace the parent theme with a clean install and make sure you haven’t missed anything (this is why you need the backup).
After that you will be able to update the parent them without affecting your customizations. It may be necessary to port major changes to your child theme if the theme developer makes a lot of updates.
