When designing a SMPS, the inductor is the most (like numero uno) import part of the whole circuit. The inductor will decide how nice your output rail is. The actual choice of inductor will change based on application, load, etc.
When selecting an inductor, you'll need to make sure your max supported amperage on the SMPS is lower than the inductors inherit saturation current.
Check out this tool from Vishay. They have very high quality inductors and are great with their documentation.
You'll also want to work with the package size of the inductor, as this will affect its inherit DCR (resistance) as well as its power dissipation capabilities. A larger package will generally have better power dissipation properties although its DCR will be higher as well (which is bad)
Many of these things are found through calculations, simulations, and such although a good place to start is:
- Pick your target output current - based on what it is powering.
- Find your switching controller IC
- Calculate your feedback loop (the feedback resistors)
- Find your mosfets (if they are needed), otherwise skip this
- Pick your inductor
Use the vishay tool to decide certain factors. Play with the parts they offer. You'll want to keep an eye on your core losses and fiddle with the switching frequency. Fiddle with the parts they offer some more. It is a good idea to de-rate the current capabilities in the back of your mind just to give your application some headroom.
- Pick your output capacitance
Your output capacitors will help filter out noise on the output rail. You'll need some low capacitance ones like .1uF to filter out high-frequency noise, as well as some larger bulk capacitors too. Bulk capacitors can be 330uF or 470uF or anything large, but it really depends on your application. Make sure these have a high enough voltage rating to handle your output voltage.
And there you have it, that is a really really really general, non-scientific way to make an SMPS.