The general idea of installing windows is to plug the big hole that you just cut in the wall
The bottom should slope out and away from the house, and the rough opening should be protected with flashing tape.
Shim the corners of the window so it will be level and then caulk the sides and top against water and air leaks.
When the window is level, nail the bottom corners.
Measuring diagonally and shimming the top corners ensures that the window is square and that it will operate properly.
Now, he nails off the perimeter and tapes the side and top flanges against water or air leaks.
The bottom is left un-taped for drainage.
And to help it along, Ben installs little shims to create a drainage channel.
The inside is sealed with special tape, backer rod, and sealant, or low-expansion foam.
—This detail was executed at Professional Remodeler's 2017 Model Remodel, built by Ben Bogie, Built to Last Design & Build.