There are various mechanisms for glycosylation, although most share several common features:
*Glycosylation, unlike glycation, is an enzymatic process. Indeed, glycosylation is thought to be the most complex post-translational modification, because of the large number of enzymatic steps involved. 
*The donor molecule is often an activated nucleotide sugar.
*The process is non-templated (unlike DNA transcription or protein translation); instead, the cell relies on segregating enzymes into different cellular compartments (e.g., endoplasmic reticulum, cisternae in Golgi apparatus). Therefore, glycosylation is a site-specific modification.