practice. that's the honest answer. no one builds a fantastic model their first time.
i've been making models for about 5-7 years, and it's just now that i'm starting to make really good ones. you need to make a few to get the hang of stuff. i find a good way to build skills is to just try stuff out, and impriovise. that's how i learned scratch-building skills. although, the FAQ section here and reading threads was a massive contibution to my skills. they really do help.
it's by experimenting that i make things like these:
5-axle custom one-off wrecker truck chassis. lots of scratch-building.
totally scratchbuilt intake plenum and fuel rail.
if you're making intake/exhaust manifold it's made a lot easier by making templates.
and finally, invaluable advice - measure twice, cut once.