When you purchase a Weep Shield product, the installation instructions should be included in the package. If not, follow this guide here:
Weep Shield fits weep-hole widths from ¼ to ¾ inch wide. Using the grip edges, simply squeeze the tabs until it is narrow enough to fit into the hole. The tabs act as a depth stop; release when they rest on the brick surface.
Height can be adjusted by cutting along the pre-formed area. The separation line is off-center intentionally.
Shorter spaces can be filled by overlapping the two pieces, or stacking the small side of two Weep Shields as shown:
Taller spaces can be filled by stacking the large side of two Weep Shields.
Almost any height variation can be covered, using these methods or a combination of them.
Note: there are some conditions where the brick-layer has left excess mortar to the point that it blocks a clear path. This is not optimal for your weep-holes, nor Weep Shield. If this happens, take a masonry drill bit and using a power drill, clean out the passage. If the blockage is severe, you may need to carefully use a chisel and hammer to remove blockage.