I really like bands like Opeth that playing really heavy stuff then suddenly switch off to mellow acoustic/clean guitar, and I want to start writing songs like that. But I have no idea where to start!

How would I go about writing Opeth-like acoustic bits?
Basically, you could try just playing the main riff of the song on the acoustic ( i find i playing it faster usually sounds better). some bands like to slightly change the style and tempo of the song a little bit, you know make it more swingy for the moment rather than a full double bass drum attack.
Seattle Will Rise Again

Vox Amp thingie I have no idea its small who cares
Fender Mexi Fat Strat ( I Stole from my friend during his F- Guitar phase)
fender jaguar HH with a Bigsby Vibrato