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it depent on what kind of SMPS.
if it is off-line SMPS ,it will be more difficult.if only from 12v to 5v,or so ,it will be a little easy.i suggest you start from the later.i did do so years ago.when you are familar with them,you can do more difficult job.
i think you can start from w*w.national.com ,such as LM2575,LM2596,ect.they have many apps and tips tell you how to select right components,deal with trouble.then you can use topswitch chips from w*w.powerint.com to build off-line SMPS.they have many doc about SMPS,and also a good software for design flayback smps.
hope this can help you.