Put your desired value into each box then click "Generate Gradient" at the bottom of the form. Code for your generated gradient will be avalable at the bottom of the page.
Colour 1 should be the darkest for the best result, colour 2 should be slightly lighter and colour 3 the lightest. The first colour option is required, the next two are optional. They can be turned off, if left on they will allow you to further customise your gradient but may restrict you in its usage.
Note that if you use colour 3 you will not be able to use the gradient as a background for an element.