Search youtube "Exo Terra tank modification for a green tree python". There is a video. He used some nice bamboo perches.
Anyway, the key is to clean the glass and perch mounts (closet rod holders) with isopropyl alcohol first. Mark where you want the perches - you can mark the outside of the glass with a crayon or grease pen. Use good silicon, the kind w/o the mold retardant is safest (although that little amount won't hurt, but still..)