This is a more detailed instruction for building the main landing gear doors. How to use the other PE parts, including the nose gear doors, should be self explanatory.The part numbers shown on the photos are for the left gear doors. Part #3 is divided into 3 sections. 

At first glue piece 12  and 27  as illustrated, on top of there respective panels. Before doing the same with #16,  I recommend to bend the upper and lower sections of the gear door in a rectangular angle away from the center section. This will give you the correct shape of the bigger main landing gear doors.You may cut off the small leg on the left side of part 12, because you will not see it anymore after building the door into place. For more reference see the first photo.