Also,Your paper should have time to expand,due to moisture in the adhesive and I would also recommend practicing relief cuts BEFORE going around window area,make your mistakes BEFORE you use your paper. Don't rush,have time to do it.
I've hung for 20 years in upscale markets,He shows good tool selection and tips, I like to use table clamps though to keep the paper from come up on the roller while pasting or wetting.. some papers pre-paste are bad and you should use a good commercial clear adhesive. Like Romans.
As you can see, this one was just about tools. I am getting ready, in about a month, to do an entire series on hanging wallpaper. I will show all you need to know.