Bas-relief (low relief) is a sculpture in which forms are slightly more prominent than the background surface. This technique involves carving away or adding material to the background surface so that the forms are only slightly projected from the background. A bas-relief may also involve painting. A base-relief doesn't have as much depth and shadow as a high relief.
High relief is the physical projection of a three-dimensional form in a painting were as the form is significantly projected from the background.
The information available here will relate only to creating a bas-relief painting.

BAS-RELIEF PAINTING
Casting  
Making Your Own Molds
Supports & Grounds
Construction & Sculpting
See also Acrylic & Mixed Media Painting Tutorials ('Lost Wing' and 'Well Balanced')
See also Air Dry Clay, Paper Making