Prairieboy's got it right. I usually up the grade of tape only if a lower stickiness won't grip. My usual litany of tapes to try, in order from least sticky/destructive to most sticky/paint-peelin': that blue tacky putty stuff, green painter's tape, blue painter's tape, paper (spiking) tape, gaffer tape, duct tape.
Depending on the window, you can also set up the gels on a frame as long as the frame won't be visible. I have a tube full of 1/4" PVC piping so I can construct such things to custom sizes as needed!
Usually soapy water in a spray bottle is used to adhere the gel to the glass. Use a squeegy to smooth it on and then push it hard into the edges and trim to fit with a sharp utility knife. Jam one squared corner of the gel into the corner of the pane and then all you have to cut are 2 sides. You might want to pre-cut the pieces to slightly larger than the pane so they're more manageable to hold and apply.