Installing new window sash's

[Cornwallrc]

I Hate to sound like an idiot, I've searched the forums a couple times but couldn't find what I was looking for.. My question is this.. I bought new window sash's (Channels at the bottom of the glass) and bought the 2 strips of rubber to use to install the new channels to the glass... But I can't seem to get the rubber on the window and into the new sash... Is there a trick to this?? can I align my window if it off a little bit?? Looking for a bit of help with this one... Thanks guys!!