It's not that hard (I've done it in a couple of remodels) but there are some things to consider. You will absolutely have to remove a lot of drywall on the inside of the wall. Window openings require a header, jack studs, cripples, and you'll likely need to remove an existing stud (or two, depending on the width of the window). And of course you'll be doing this to a load-bearing wall so some temporary support is likely called for.
Cutting the exterior siding is actually the easiest part. Once the window opening is framed in, just cut around it with a sawzall.
Cutting the exterior siding is actually the easiest part. Once the window opening is framed in, just cut around it with a sawzall.