Look where you are shooting and examine the scene. Determine what windows will be throwing light into your scene.
Use your measuring tape and take measurements of the window lengths and widths making sure to write them down as windows may be different sizes.
Pull out your cardboard and refer to your list of measurements.
Using the measurements of the windows begin drawing them onto the cardboard but adding 2 inches to each side as well as top and bottom.
Cut out the cardboard along your lines.
Cover the cardboard with the spray paint and allow to dry completely.
Line the cardboard up with the window so that the two inches can be visibly seen around the entire window.
Using duct tape begin taping the cardboard to the surrounding wall by keeping half the tape on the cardboard and half the tape on the wall.
Add a second layer of duct tape so that half the tape is overlapping the wall edge of the previous used duct tape and the other half is on the wall.
Repeat Step 9 but on the inside edge of the cardboard.