With concrete block home construction, window and door opening accuracy is much more important that in wood frame construction. With wood you can cut out a little if necessary, or move a wall stud when required. A surface bonded block wall can't be easily changed. You must be sure where you want that window and door opening early in the planning phase.
Using a jig for square window/door openings.
We now recommend that you stack the wall rows until you reach the block row which will have the window sill (as shown below, that is row three). Now place at the correct wall location and shim the window jig so it is level on that row of blocks and vertically plumb and brace the jig to the ground. Now stack the blocks against the jig. Carefully place the first block on each side of the jig so the jig is not moved horizontally. The closing blocks for each row can compensate for any window opening variance. Remember that one side of each window/door block row will most likely have a half block.With a door opening, just place the door jig on the floor between the door-opening mortared blocks as they are mortared.
When the wall is completed, and the wall cells are poured and cured, pour the window sill before setting the window header forms. Don't forget to place a concrete stopper (Chapter 15 in the eBook) under block row three in this example. The window sill height should place the top of the windows nearly in line with the top of the doors. Now place the window jig on this overnight-cured window sill, and build the window header forms for exact height. Place window header board props on the poured window sill (Chapter 14 of the eBook), so that the jig fits into the window/door wall opening, and pour the header. This will result in a window/door opening that is square, plumb, and makes later window/door installation a breeze. Cut a couple of large holes in the jig bracing plywood to allow for easy handling of the jig. The cheapest jig can be made from OSB (just keep it dry), cut to be exactly square and of the desired wall rough opening dimensions.
This diagram does not indicate whether one or two rebar are required in each location. Check your local building code for exact dimensions and requirements in your area. You will find mention in the code about the rebar being 2-1/2 inches above the bottom of the header pour. This reference is with respect to the minimum header height as defined by the empirical design tables.
Since my empirical design requirements call for a 12 inch high header, then my bottom rebar has to be 9-1/2 inches below the top of the header. Since my lower rebar is about 15 inches below the top of the header I have exceeded the strength requirements of the header span. The additional concrete below the lower rebar does not matter since it does not contribute to header strength and only serves to establish the top of the window opening. Refer to the door framing section below for door opening concepts.
In this same picture, you can see that the bottom window sill is also concrete. I used precast window sills in the garage because I just didn't know any better at the time. For the house, I will use poured in place window sills for a couple of reasons. In the house construction, the poured in place window sill can be height adjusted to exactly match the required window vertical wall opening. Once the wall is SBC'd, then the final window opening will look very professional and will accept the window rough framing quite easily. SBC doesn't stick to precast concrete without a special additive. By pouring concrete window sills, they can be SBC's right into the wall without resorting to a special SBC mix. You could also use narrower window framing boards like I did in the garage, if you want the window recessed into the wall. I used 2x6s in my garage to have outside recessed widows that are flush on the inside.