Landscaping - Creating a slope

We are in the process of renovating our backyard, and I would like to create a slope in an area where it is presently composed of a flat yard with a four foot wide tier that backs up into a wall, and is about one foot above the rest of the yard. The final result we want is a slope that begins at the wall and descends down at about a twenty degree angle to join and flatten out with the lawn, and landscaped with foliage and ground cover. We have plenty of loose earth to create a slope, but I am concerned about the final slope not being stable, sliding, etc. Any suggestions about how to build a stable slope such as this?