Installing a window isn't difficult, just make sure you understand what type of window you're working with. These instructions are for a window with no nailing flange. Most unflanged windows are made of wood, and most have brick molding.
When ordering a window be sure to specify a jamb of the correct width. If your walls have 2x4 studs and 1/2-inch exterior sheathing, the jambs should be 3 5/8 inches wide (the extra 1/8 inch allows for imperfections). For a wall with 2x6 studs, a jamb width of 5 5/8 inches is correct.
Unpack the window and inspect it before installing. Make sure all the weatherstripping is in good shape and the mechanisms and sashes operate smoothly.
Check with your building department to find which method of wrapping the opening is preferred; some departments have stringent requirements.
Once the opening is framed, expect to spend about 3 hours sealing the opening and installing a window and its exterior trim. Also allow time for finishing the interior wall and installing interior trim.