A combined coarse-graining and back-mapping approach to preparing relaxed systems of amorphous polymers with atomistic detail is presented and applied to the case of bulk amorphous cellulose. The coarse-grained model is first developed using results from a standard molecular dynamics atomistic simulation of octaose, i.e., the short 8-ring oligomer of cellulose. The change to a coarse-grained scale leads to an effective speed-up in excess of 2000 for the computational efficiency of the relaxation of the chains. A representation of one ring by one bead is used in order to maintain a good description of the envelope of the molecule and this allows for a subsequent seamless reintroduction of the atoms. The back-mapping procedure has been successfully tested on dense bulk systems of both octaose and hectaose, i.e., 100-ring cellulose chains. The principal advantage and motivation for this approach is that it adapts well to cases where interfaces are present and spatial isotropy of chain conformations is no longer assured or easily predictable.