I won't explain how to build a great architecture, I just can give some tips how to learn it. Because it's just impossible to explain architecture.
People always complain when you rip off other maps, but I have no idea why.
That was the way I and many others I know learned some architecture stuff. My
first maps were just some cubes stuck together,
then I built a map in the style of dig and dug (the mine maps in Unreal), I looked at Cliffy's architecture in these maps and just built the same. That stuff helps a lot, because you'll understand why he did do it like that...
When you are playing maps, always have an eye on the architecture, try to understand why it looks good (if it does) and try to copy it and maybe improve it a bit.
Once you've "ripped" several styles you'll start developing your own architectural ideas.
If you cannot build something, though you know what it looks like, load the map in the editor and look at the brushes...now everybody should be able to build it (presupposed you know all functions of the editor).
Something that helps as well.....in real-life, when you see an object, try to see it's wireframe, try to build it of the editor primitives in your mind....
Most people start bitching at you when you release your rip-off work, you
don't have to release it, but if possible show it to the author of the map that
you've ripped and ask what he (are there female mappers btw?) thinks about it,
maybe he can provide some hints.
Well.....yeah, I haven't got anything else to say about that. If you have questions ask me...firstname.lastname@example.org