Creating your new Genesis subtheme is a simple three step process. In short just copy one of the included starter themes to your themes directory and rename the .info file and give your theme a unique name and description (done inside the info file), then rename the preprocess functions in your subthemes template.php file. After that you are ready to upload, enable and set to work.

What follows is just a more detailed version of the above instructions.

1. Copy and Paste one of the Starter themes

First make a copy of genesis_SUBTHEME or genesis_LITE.

Place the copy in sites/all/themes.

Change "SUBTHEME" or "LITE" to your theme name e.g. genesis_mytheme

2. Rename .info, then add your theme name and description

Next, rename genesis_SUBTHEME.info to genesis_mytheme.info

Inside genesis_mytheme.info file change:

i) The theme name.
ii) The description.

3. Rename the preprocess functions in your subthemes template.php file

Note: this file is inside your subtheme folder, e.g. genesis_mytheme/template.php.

For example:
function genesis_SUBTHEME_preprocess_page should be renamed to function genesis_mytheme_preprocess_page, where genesis_mytheme is the name of your theme.

Now you are ready to start theming.