Flyback Converter Design Process
I would like to warn you that full blown mains converter design, at any reasonable power level is not for beginners, far from it. The mains AC poses high risks during the design and debugging process and there are many practical pitfalls both in schematic design and in the layout of the design, such as parasitics. 

But regarding the question the design resources are quite scattered. I do not ever remember running into a "A to Z" article for a multiple output mains converter with all the important aspects covered, but i can tell you that many datasheets and application notes from the various IC giants (Texas Instruments, Analog Devices, Onsemi, Maxim, etc, etc) contain various information in varying quantities. So unless you are willing to buy a good book on converter design (many available from amazon for example, i do not know the best one) you should look at as many PWM controllers datasheets as you can find, you might even run into a controller that makes your life as easy as possible.

You will run into another challenge when you look at the feedback loop design, especially if it requires manual compensation, but you will find some articles online on different compensation schemes and how to use a TL431 for example to create a feedback path through a optocoupler, but it is alot of guesswork and very difficult to verify without a network/spectrum analyzer.