I am getting ready to install replacement windows and need to measure my existing opening to get the right size windows. I have seen numerous instructions online, but they are not very clear. My existing windows are aluminum. My first question is, does anyone know any websites out there with good instructions. My second question, do I need to remove my window trim to measure the opening? Thanks for all your help.

Bab, My advice would be to let a replacement window outfit(not a manufacturer,just an outlet store, like Menards,Or a local lumber yard who deals with replacements) come to you to measure the replacements. I do this for this reason,If the measurement is wrong,they are responsible for the replacement,not me,in case I measured wrong.(and after 20 years in remodeling, I still do measure wrong sometimes)

Hope this helped,

KBC