I just replaced one of these yesterday, along with a brand new screen (£18, "A grade" screen, none of this cheap rubbish!). I bought the iPod Touch 4th gen for £50 with a cracked screen, and when I disassembled it to replace the screen, I tore off the volume (+) (-) / Power ribbon cable, so had to buy and fit a brand new assembly for that too (£2.40).
They're certainly fiddly to fix, but not impossible, and very well made. The power button has a solid die cast metal plate behind it (very thick) and you simply peel the old tactile dome and ribbon ass'y from it, scrape and clean the glue off, peel the ribbon right back off the inside back casing, unscrew the volume control casing, remove that, then peel off the old volume control ass'y (it's on the other end of the same ass'y, as shown in the packet on the right hand photo), and fit a new one, which is affixed with VERY strong 3M backing.
The reason I did this, is because Apple wanted £82 to "repair" (replace) it for me, which, on top of the £50 I paid for it broken, was ridiculous, and why pay them £82 for an out of warranty "repair", when I can do the whole lot for ~£21 myself?

It's working superbly now.