Quote Originally Posted by ZuruiChibi
I was just looking at the 12 bar blues progressions. Looking at the chords involved, am I right in thinking that it's basically a musical phrase stretched over 12 bars?
Not exactly. It's 3 musical phrases; or rather one phrase (4 bars) with two variations. Each 4-bar phrase has two halves, the first half (2 bars) mostly vocal, the 2nd half mostly instrumental response. In terms of the chords:

1. I-I-I-I
2. IV-IV-I-I
3. V-IV-I-I

That's the underlying format, but most blues songs add more detail to that.

In most blues, the whole 12-bar section is a "verse", and each song will have many verses.
Typically there are 3 lines of lyrics per verse, with the first 2 lines being identical. Naturally these fit the three 4-bar musical phrases, as described above. But sometimes the first 4 bars will consist of 2 shorter lyric lines.
(There's all kinds of variations - lyrically and structurally - on this basic format.)

In some pop and rock songs (influenced by blues or rock'n'roll) you might get a 12-bar verse with a chorus in different format - or vice versa.
Quote Originally Posted by ZuruiChibi
And for a 24 bar progression a repeating phrase, with a slight variation?
A 24-bar blues is usually just a 12-bar with everything doubled in length. IOW, still 3 distinct phrases or lines, but each 8 bars long instead of 4
Quote Originally Posted by ZuruiChibi
I have another question. How do you turn these progressions into songs? I get frustrated because nobody explainst this. There are lots of vids and articles on 12 bar progressions, all saying exactly the same things! And no mention of how to put it all together in a song.
Well, you think of some words and sing them! If it's blues you want to start with (and it's an excellent classic starting place), just listen to old blues songs. I mean almost any blues pre-1960. And from Chicago or Memphis...
Quote Originally Posted by ZuruiChibi
I'm looking at Guns 'N Roses at the moment. I'm not that fussed about their music, or even the blues for that matter! But I like how they embellish their progressions. However, the blend between riffs and progressions confuses me alot. Especially when looking at structure. Does anyone have an insights into how they put this all together?
G'n'R are actually pretty sophisticated songwriters. Their structures can be very complicated. They're not the best band to look at when learning songwriting - it's like dipping into the book around the middle somewhere.
Chapter 1, as it were (if we're talking pop/rock songwriting) is people like Hank Williams, Buddy Holly, Chuck Berry, Lieber/Stoller, and the tin-pan alley writers like Phil Spector, Goffin/King, etc. IOW, the 1950s and early 60s. Essentially a mix of country and R&B, with some elements of jazz standards and Broadway musical heritage.
Then in chapter 2 you get the Beatles (who really deserve a chapter to themselves ). They basically put all the stuff in chapter 1 together, mixed it up and spewed out something inspiringly "new".
Chapter 3 might be the Tamla writers, Holland/Dozier/Holland and Smokey Robinson.
Chapter 4 could be Bob Dylan, who opened up the lyrics and expanded the pop writer's consciousness. He made it cool to be serious. (Chapters 2, 3 and 4 all deal with the same period, roughly.)
Chapter 5 would be the rock bands of the late 60s/early 70s: Hendrix, Floyd, Zeppelin, Zappa, etc. The way they opened things out instrumentally, extending songs with long vamps, more varied sections, more improvisation opportunities. Prog rock grew out of that, often developing self-consciously overblown structures, seeking "classical" approval. (You could have a separate, short chapter on prog.)
After that would come the great pop/rock writers of the 70s: Bowie, Springsteen, Elton John, etc. More depth and complexity, often more extended structures, but retaining a tight pop sensibility.
Punk represented a return to the simple values of the early/mid 60s, and kept most subsequent rock music with its "feet on the ground".

As I hear it, rock music after that (80s, 90s, this century) all builds on various elements of that past, with very little noticeably new. (We're still talking songwriting here, OK?)

The point is, if you want to understand rock song writing, you need to appreciate this history, and look back to the earliest song structures, which were little more than 12-bars (if derived from blues) or 32-bar AABA (if derived from jazz, Broadway musicals or country).
Many (if not most) contemporary rock songs can still be analysed in these ways - particularly the AABA template, with an opening "A" section repeated, followed by a "bridge" (B), followed by a repetition of the opening A. Lots are even the same old 32 bars (8 bars per section), tho many will add intros, interludes, and secondary bridges ("C" sections).

As for what inspires specific bands (or specific songs) that's harder to say. They all work within the tradition, more or less conscious of the past.
Some will have a philosophy of "keeping it live": experimenting in the studio, but not getting too complicated. Others will luxuriate in extended studio time, piling on tracks and details because they can: constructing an impressive audio artefact, IOW.
Some - esp individuals who write on their own - will come to a studio with finished songs, and just get them down with little amendment. Others (such as bands that write collectively) will vamp and improvise, pulling random ideas together on the spot. A singer will have some words, the musicians might have a riff: maybe they'll slot together. Maybe playing it through a few times will spark other ideas.