The rules are simple: finish the line or write in the next one, then post the beginning of another line from another song, e.g.:
There’s a lady who’s sure all that glitters is gold…
…and she’s buying a stairway to heaven.
Yeah you know when I drink alone…
…I prefer to be by myself.
(new song lyric here)…
and so on.
Just try not to make it too obscure, and post enough of the starting lyrics so as to not be impossibly ambiguous.