Also, in case it isn't clear:
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PC ATX power supplies are all switched-mode power supplies - this is why they are so lightweight (no large iron core transformer).
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The power supply schematics you see online are almost all linear power supply designs (hence the large and heavy transformer).
Linear supplies tend to be easier to design than switched-mode supplies. Even so, a linear supply isn't a beginner's project, but a small one (like you are wanting) can be a great way to learn the basics first. As others have noted, you won't be able to use any of the transformers from the ATX supplies (as they are meant for switched operation).
Once you have the concepts of a linear supply down, then you can move on to switched-mode supply design (which will likely mean needing a dual-trace oscilloscope as one of your tools, btw).