Find a local DG company, pop along with your preferred sizes and ask if they have any mis- measures, pick the best size that suits your needs, make sure you get the glass with it and a cill, take it home, measure window and transfer size onto block work preferably lining one side up with the vertical mortar joints so you have less cutting and allow an extra 10mm, remove blocks while supporting any that are above, depending on the size of the window you may need a concrete lintel or some angle iron and overlap 6 inches at each end.
Place window in opening, pack it square, fix it using frame anchors or spax concrete screws (other makes are available), fit glass and seal up!
