Preparing the opening and window
As you can see the mortor is extending well into the opening and will
have to be chiseled off flush with the brick. Also cleanup as much of
the old caulking as you can being careful around the top few courses of
brick so you don't knock them loose.
Using a cole chisel clean out the mortor near the top and bottom of each side
of the opening, tall windows will need a block near the center also.
Then install 3/4 inch exterior plywood blocks flush with brick or held slightly
into the opening and fasten securly with screws.
Notice the difference in the locations from the top left to the top right because
of the longer soldier bricks on the right. This will work fine.
Drill 3/16 inch holes in sides of window frame to correspond with blocking.
Countersink each hole so sash does not hit screw head.
Measuring for replacement windows
Removing the glass