You could immerse the glass in oxidant acids (~20-30% H2SO4 or HNO3) for several hours (6-12 h) at room temperature. A quicker way to do it would involve treating glass with an oxidant solution such as "piranha etch" ( 3 parts of concentrated sulfuric acid and 1 part of 30% hydrogen peroxide solution).
Usually polishing the glass beforehand (2500 grit) helps.
Much depends on the way of grafting you want to use. Si_OH groups are already there. If you use silane treatment with a reactive fictional group you may graft the polymer chain to the silane layer.