Here are the basic rules:
1. write a poem composed of lines from other poems (or song lyrics)
2. don’t change any of the original words (I didn’t follow that rule in my first one)
3. cite your sources so that we can read the lines you chose in their original context.
From the OED (online):
1. A piece of patchwork; a patched garment.
2. ‘A composition formed by joining scraps from other authors’ (J.).
b. more loosely: cf. ‘string’, ‘rigmarole’.
3. transf. (of persons, etc.) Obs.
Hence centoism (also centonism); centonical a., of the nature of a cento; centoize v., to make into a cento.