I used that 70's style glaze in the pic for my rear window as it matched what came out of mine. We didn't trim off the flanges but used an automotive silicone lubricant to help break the friction and make it slide, in my opinion those flanges are a very big part of what is doing the waterproofing for the window and I would avoid doing that at all costs. We used the silglide on both sides of the gasket, the part that touches the glass and the part that touches the window channel.
When it comes to cutting the gasket, cut it slightly longer than needed as it will shrink way quicker and more than you would think, forcing some extra lenght in the it will accommodate for the shrinkage. Being that my rear window is rectangular I had to miter the corners. I forced about an extra cm into the horizontal edge and 1/2 cm into the vertical. After it was all forced in to place take a bunch of elbow grease and paper towels to get the silglide off the window.
To finish the off force Sitkaflex 221 Grey adhesive into the miters, or seams if you're working with the rounded style windows :-)