“Converting” is more like the process of having it automated (by Photoshop, for example), but this usually requires a lot of cleaning up and fixing, cause the output is usually very generic. That’s why I think @fordinary mentioned it was old fashioned.
To do it yourself is not something we can explain to you right away. Like you said, each design is different and requires different techniques, but it’s something you’ll have to get skilled at.
Myself, I just print the PSD on paper and then I start with a blank HTML file. I create some basic blocks on the page (header, main block, sidebar, footer, whatever other major areas a site has) and start styling them. Then I fill in details.
Sure that sounds super generic but that’s really what it is….and Chris’ screencast explains it a thousand times better.
If you have issues with anything specific, feel free to ask for help. But to be honest, a question like “how do you make a web site from a design” is a little bit too generic, if you know what I mean.