prepare the replacement window by first removing the orange packing clips next raise the lower sash and remove the insect screen set it aside in a safe place to avoid damage before installing the replacement insert window clean the opening with a brush or vacuum to remove any dust and debris and dry fit the replacement window before applying sealant to the windowsill after verifying the fit apply a bead of sealant along the
entire length of the sill right next to the stool run a second bead of sealant parallel to the first bead but two inches apart finally run a bead of sealant at each side of the window sill and extend out to the edge of the exterior casing when lifting window units use a reasonable number of people for the size of window you're installing and always use safe lifting techniques with both sash in the
upper position lift the window into the center of the opening sill end first tilt the top of the window up and pull the unit tight to the stool at the sill and tight against the outside head stop to ensure positive contact between the sealant and the sill fasten one of the provided two inch screws at each side through the pre-drilled holes in the lower section of the frame drive them in just far enough
that they don't impede the operation of the sash but do not fully tighten the screws at this time lower both sash and fasten two inch screws through the pre-drilled holes at the top and center of the window frame again do not fully tighten the screws check the unit for square by taking diagonal measurements from corner to corner...
See more here: https://www.youtube.com/watch?v=HNK_w-8L2io