First, remove the window panes from the window frame.Then, measure and cut the glass blocks to size using a wet saw or masonry saw. (It's best to use a wet saw if you're cutting more than a few blocks, as it creates less dust.)Next, apply a layer of mortar to the back of each block using a trowel. (Make sure the mortar is thin enough that it won't show once the window is reinstalled.)

Well, it depends on the window. If it's a standard double-hung window where the top sash slides up and down, then you can remove the top sash and drill through the side of the upper frame to secure the glass block in place. if it's a casement window (a window that opens out like a door), then you would remove the entire window frame and replace it with one that has been specifically designed to hold glass blocks.Hope that helps!