Air leaks in your home are the death of your HVAC system's efficiency. Unfortunately, windows are a common source of leaks. You can mitigate some of this risk by installing the window properly in the first place. If you have that, doing your regular maintenance will successfully keep your home as efficient as possible.
Prep The Wall
The key to getting a good seal on vinyl windows is to prep the opening properly. The phrase "measure twice, cut once" is extremely accurate here. Make sure to leave enough space for the window, the framing and the flashing. If you cut the opening too large, it is repairable, but you will lose some of the structural integrity.
While you are measuring, use a square, as well as a plumb to ensure the opening is perfectly square and level. If you do any kind of remodeling regularly, than a square is a vital investment. A plumb is essentially a weight hung from a string, so you can make this device yourself whenever you need it. Hang it from the top corner and you will be able to check if your sides are actually going straight down.
Once you make the cuts, a level will assist you in making sure each step retains those straight edges.
Install the Flashing Correctly
Once you have cut and framed in the hole for the window. It is time to install the flashing around the frame. This protects the core of your wall from any moisture that gets inside the siding. While you might get a good seal on the window initially, over time, the water will eat into the wood. Initially this will just cause air leaks. If it goes unrepaired long enough, you will end up with structural issues within your wall.
Just as with any other flashing installation, this is all about giving yourself plenty of overlap. Make sure that the window both fits over the sill and hangs down below the siding. Nail it down securely, and use plenty of caulk to finish the nail holes and to close any gaps between the pieces.
Follow the directions for your window to ensure that each side of the window has the correct style of flashing. Usually, the bottom of the window will need stronger flashing than the other sides since this is where the water would pool if it got in.
Finish the Installation Properly
The precious steps should take care of sealing the window from window and air, but it isn't the end of the story. You still need to reinstall the siding and frames around the window. While they aren't part of the core, they can still destroy the layers beneath. This is just a matter of being careful with your installation so that the underlying layers don't get torn up.
In addition, you should consider future maintenance in your plans. If you can make it easier on yourself in the future, why wouldn't you? You want to be able to have these windows for the long haul, so consider leaving yourself space around the window to properly clean and caulk it.
Installing a window is a big and heavy job, and it certainly isn't for everyone. Do your research and talk to a few people who have done the work before you make the commitment. There is nothing wrong with hiring an installer. They have been putting in windows for many years, and that decision will ensure that you get a window that is installed quickly and sealed properly. If you do decide to do your own installation, just be sure to take the time to do it right the first time.