First, make a drawing of the bag you have in mind or seek a picture of a bag you like.
Second, find a pattern you like or design your own pattern!
Third, start and make all elements, add them together and finish the bag!
Fourth, Wear your own handmade bag and be its proud owner.
An example of the whole proces of making bags can be found in Cthulhu-handbag-pattern
Making pouches is also very nice to do. I have already written a page about it read more on bags-and-pouches.