Well you need to sand flush any bumps and scoops on the nose, cowling, and upper wings. This includes the large bulges for the machine guns on the upper cowling. Hopefully, the kit provides these as separate pieces, otherwise you are in for much work to remove and fill the holes that remain.
The next major issue is the canopy. The 109F canopy has glass panels on the lower front corners and the 109G has sheet metal in those places. The kit uses the plastic molded to the fuselage in this area. See this picture for a 109F canopy:
Also you will want to represent the hatches seen in the above picture as they are not on the G model. The most important one is the large hatch below the canopy, which is for the fuel filler. There is a small, oval shaped, hatch on the upper left of the rear fuselage that needs to be filled - as that is the fuel filler on the G models.