Start laying your block and create one continuous row including where your openings will be. Make sure it is square on all sides. If you purchased the half block corner sashes keep in mind that one will be placed on either side of the block that is in place of your future opening.
Next, lay your second row using full size block and using the sash units at your corners. You don’t have to place a block at your planned opening. The first block laid is simply a guide and will/can be removed.
The third row will be laid the same as the first, including the half blocks. It should look similar to the pictures below.
