well, I would get a isolation transformer, a differential oscilloscope probe, then this
http://cds.linear.com/docs/en/application-note/an25fa.pdfbuying it would probably be 30000 times easier, or using a transformer.
page 8
if you wanna build it, and sell it, your gonna have to worry about EMI protection, radiated noise, compliance, input protection. It is a pretty in depth design to make a mains high voltage not really isolated switch mode supply, and someone else will need to test it for certification, probably multiple times , unless you have nice equipment and know what you are doing, so you only need to test it once.. if you make your own you will defiantly need a spectrum analyzer and some special "jigs" that interface your circuit to the spectrum analyzer, at the very minimum. you will also probobly need some kind of free space probes to measure radiated non conducted interference.
you cannot get away from sending it out for compliance testing, the best you can do is test it yourself and fix errors that the compliance people will have an issue about
i would just plagiarize something that already exists instead of trying to design one, i think this has quite a few traps for young players.. non trivial.
I want to one day design my own, for designs sake.. it certainty is a interesting challenge that I will take over sodoku puzzles.