trick to install GG&G rail covers?

[Falstaff]

I got one on, but it was very difficult. I simply cant get the others to go more than halfway. Are you supposed to slide it on endwise (sliding down oparalell to barrel) or sideways (hooking one side of the rail cover over the picatinny rail then snapping it over). the lugs inside the railcover seem to be keeping it from sliding down..