Steps of customizing an existing theme can be broken into three major steps
Select the base theme.
Create a sub-theme from the base theme
Make the changes to the new sub-theme
The relationship between a base theme and a sub-theme is almost like a parent-child relationship in object oriented programming. The child also called sub-theme which inherits its properties and characteristics from its parent also called the base-theme. A sub-theme can be the child of another sub-theme. There are no limits to chain together multiple parent-child relationships.
Never ever touch the original files provided with download, it is because when we upgrade them we won't be losing our customizations work.
Steps to Create a sub theme, taking bartik as parent theme
Copy the bartik entire theme folder and paste it in /sites/all/themes
Rename the bartik folder name to eg bond007
Rename the bartik.info file to bond007.info
Delete all the file under the templates folder except the page.tpl.php
Delete all the file under css folder
Replace the occurrence of `bartik` in file's inside the bond007 folder and with bond007
Sub-themes do not inherit any custom regions defined in parent theme. We specifically need to re-specify them in sub-theme, in our case we need to redefine the custom regions in bond007.info file. If we do not declare regions at all, we will get the following regions by default: header, help, highlight, content, sidebar first, sidebar second, and footer. and also we need at least one stylesheet. As this is a requirement for a valid sub-theme.
By now you are ready to customize the bond007 theme according to your need.