Building A Concrete Block Wall

As the block, I'm assuming that aesthetics are not as important. Then go to the lens of steel above the opening. The blocks on the sides of the assistance to the opening of the steel and steel supports Slug above.

If you're willing to go to aesthetics, your local store supplies masonry can sell you one piece of concrete lentils to go above the door.

For the size, leave an opening large enough for your rough opening your door frame, with a margin of spare on each side for setting. Once the wall and lentils are done, go back and hold in the frame, and set the framework for the blocks following the instructions door (you'll need for mechanical fasteners of the frame to the block), then mortar around the frame to fill the gaps in the air.

Make sure the door works, before the mortar in the frame.

Leave openings, rather than "set". You know what the size of these openings, so just build vertically whenever you want. You'll need lintels above to take the weight of the blocks above them / You do not say if the cavity wall, bearing wall or just a wall of separation, so I can not comment on the lintel you need at this time.

Usually you part of the opening with wood, the shape of the block around the frame, then knock out the wood and the installation of the window. Pour pack cells around the window gives you a better anchorage.