If you want to do it by yourself, here are some tips :
- try to figure out the different elements of your website… for instance, you can separate 3 type of graphics :
- images that you will need to slice as they are (header, static blocks...)
- images that will be part of a CSS style (background image that will repeat horizontally : you’ll only need to slice it 1px wide)
- parts that can be done in full CSS (agrey block with only a black border can be done in CSS… a menu item or a button witn only a background color and border color can be done this way too...)
- then create a new theme for your Magento site (browse the Designer Guide for detailed instructions, or copy the same folder structure from the default theme to your new theme).
- play with CSS in the boxes.css stylesheet mainly.
Don’t hesitate to ask questions if needed !