Slide the blade of a putty knife beneath the edge of the vinyl. Lift it from the window, taking care not to scratch the glass with the knife. Peel away as much of the vinyl as possible, using the putty knife to help lift and dislodge it as you work.
Aim a hair dryer at any portions of the vinyl that resists lifting. Turn the dryer on the highest heat setting. The heat from the dryer softens the adhesive so you can get the putty knife beneath the edge of the sticker.
Scrape as much excess adhesive from the glass with the putty knife as possible, after the vinyl is removed. Hold the knife so the blade is nearly flat against the glass as you scrape. This allows the knife to glide over the glass so it doesn't gouge the thin protective coating on top of most auto glass.
Soak any remaining adhesive in white vinegar, rubbing alcohol or a commercial adhesive remover. Scrub the soaked adhesive with a rough cloth until it is removed completely.