Not real hard. They are glued in and also has tiny little plastic keepers. Some will not come out so you will defiantly tear up the old stuff getting it off.
I pick an end and go from there. I used the Black RTV sealer in spots. Some use it all the way around. The upper gasket is more of a problem. Getting it all out of the track (drainage channel) is somewhat cumbersome.
I'd put the gasket on dry then once it is in place and your satisfied with how it sits and all then use the black sealer or the yellow. Be sure to have the door closed and roll up the window to see how the top gasket fits and seals. You'll be able to adjust it some before you put the adhesive on and will possibly need to.