The next time you are in your living room, sitting on your sofa you may want to know more about how a sofa is made. Here are the basics, but first this excellent time lapse video shows modern sofa construction in a workshop in the United States.
Generally, you can judge the quality of a sofa by the frame so this is something to look into. It should be solid and sturdy to ensure durability. Kiln-dried hardwood would be your best bet because it will get rid of moisture and be able to keep it shape. The joints should be glued or screwed in well. The corners must be strengthened with blocks. If you are using plywood then make sure you use a couple of layers to give it extra strength.
Having the right kind of spring system is important here. These are connected with a strong twine. Springs are arranged in different ways. You will find them in an "S" shape or in the form of a web suspension where you will cover the frame and make the right kind of support for the cushions to be placed on top. The more conventional way is to just tie the twine side to side.
This video is a little more sedate than the one above!
The cushioning will depend on what you want out of your sofa. If you want something soft, then down would be your best option. Of course, you will also have to think of your budget and there is no doubt that down is one of the more expensive types of cushions out there. It is also is true that quality cushions are going to last a lot longer in the long run. You can also use a combination of different materials and this is going to cut down the costs. Using dacron polyester pads is a good choice because it won't lose its shape easily.
Again, this will depend on factors like your budget, quality and durability. Slip covers have become really popular these days and it seems like common sense because you just have to unzip them and throw them in the wash. A beautiful white sofa can be ruined by a glass of red wine so these are things to keep in mind.
To continue your education in all things sofas, have a read about sofa history.