The chemical reactions and pathways resulting in the formation of O3-(phosphoglycosyl-D-mannose-1-phosphoryl)-L-serine. The polypeptide backbones of glycoproteins and mucin-like proteoglycans are extensively modified with a complex array of phosphoglycan chains that are linked to Ser/Thr-rich domains via a common Man-alpha1-PO4-Ser linkage.