Got The Right Size! Now what?

I measured my existing snap-in and ordered the right size. Hooray...95% of the battle.

My large window is a series of smaller,individual panes that are 11.5" width by 13.5" H. Each pane will need to have a Snap-In cut to accommodate 11.5" window width in order to install it.

Can you give me some guidance on installation? Is there a preferred way to cut and install the snap-ins? Do I cut exactly 11.5"? Do I need to cut at an angle? Thanks!