Nail a strip of 1" by 2" wood along the center of the entire rough opening to support the weight of the blocks (Image 1). Attach strips of water-resistant drywall on both sides of the 1" x 2" to finish the opening (Image 2).
Apply a coat of a water-based asphalt emulsion to the 1"x2" wood on the sill of the opening to protect the wood from moisture damage (Image 3). Don't apply the emulsion too close to the edge because the glass blocks will leave a lip, or "reveal," around the perimeter of the opening.
After you've prepared the openings, you'll install the panel anchors. Bend the panel anchors at a 90-degree angle, 3" from one end. Attach the panels to the jambs at a height equal to that of a stack of three blocks. You may want to add 1/2" to compensate for the mortar, depending on how much mortar you plan to use. Attach another panel anchor for every third row of blocks. The anchors will help secure the blocks to the jamb.
Glass-block mortar is made specifically this purpose and is drier than regular mortar since glass absorbs no moisture. To test whether the mortar is dry enough, press a handful against the side of a glass block. If the mortar stays in place, it's ready to use.
Dry-fit a row of the blocks using plastic spacers. Cut the spacers for the edge and corner pieces so that the sides against the wall will be flat. When you're comfortable with the way the blocks will fit together, begin laying them with mortar.
Lay a line of mortar on the sill, using enough so that it will squeeze out when you set the blocks in place (Image 1). Don't apply mortar to the jambs. Set the spacers in place, then set the first glass block in place.
For the next block, apply mortar to the side that will be closest to the adjacent block (Image 2). Again, apply enough mortar that it will squeeze out when you press the blocks together. Repeat the process until you have completed the first row of blocks. Check the row for level and plumb. Wipe off excess mortar with a damp sponge (Image 3).