Set the first row by placing whole bricks across the bottom (Image 1) and a half-brick if necessary. Since this is the first course, it's the most important.
To create an offset pattern, the next row will begin with one of the half-bricks, followed by two whole bricks and completed with another half-brick. The trick is to be sure to place the cut side of the brick toward the back wall because when it's mortared you'll never see it.
As you continue, be sure to leave about 1/2" of space between both the horizontal rows and the end of the bricks vertically. You can do this easily by using the end of your fingertip as a guide. Repeat the pattern working your way up the wall.
Finally, install the brick onto the unfinished inside edge beginning from the bottom, allowing it to extend past the edge of the alcove opening by 1/2". Use tile nippers in order to create a perfect fit (Image 2).