Are you talking about the standard DVD slip case? I have one right here...
OK, since I'm helping you I'll restrict my discussion to using metric units.
All of the following advice is assuming that you are printing this out yourself instead of sending it to a printing firm.
1. Set Photoshop up for millimetres (mm) in preferences.
2. Create a new document 270mm W x 180mm H at somewhere between 300-600ppi or even higher depending on crisp you want your type to be and how powerful your Mac is. Work in RGB.
3. Show the ruler (Apple-R). Pull two guides out, one at 129mm, the other at 142mm. This is the spine of the cover you can run text vertically in this space if you like. Run very fine and short lines on the guides top and bottom these are your folds.
4. Do your artwork, save a copy and flatten the artwork before converting to CMYK, a little unsharp masking and then printing centred on A4 and trim. Score and fold along the lines you created in step 3.