This is a very easy and effective tutorial.
Start a new design.
1. Draw a box 5 inches by 5 inches and extrude by 5 inches
2. Select the top face with the select faces key
3. Go to Feature > Modify Solids > Deform Face
4. You can now drag the small yellow box to adjust the curve on that side. Or type in a number which will move the yellow box for you.
This is very effective and can be used on any flat side.