Every installation is different depending on your home construction, the condition of the opening, the type of products you choose and the materials used.
FULL FRAME WINDOW INSTALLATION METHOD
Removing the entire existing window and installing new.
A full frame window installation makes sense when:
The old window frame is deteriorated beyond repair.
A larger opening is required by fire egress codes.
You want to improve your home's energy efficiency, because the old window frame has air leakage.
Existing window frame doesn't lend itself to insert window replacement.
INSERT WINDOW INSTALLATION METHOD
Setting a new window into an existing frame opening.
Insert window installation makes sense when:
The old window sill and frame are square, solid and in good condition.
The existing construction will allow use of the insert in a way that is structurally sound.
Casing and trim need to remain in place throughout installation.