I've had a good look through of the Deathwatch core rule book and the supplements Rites of Battle, First Founding and Honour the Chapter, so I've got a pretty good idea of how to build my own chapter. Yet one thing that is not addressed at all is the variations on the primarch's curse for each chapter. You can see the changes in successor's like the Flesh Tearer's chapter from the Blood Angels, or the Black Templar's from the Imperial Fists. But if Iwant a successor chapter, I gotta have some way of differentiating the gradual changes of my battle-brothers over time from their progenitor chapter.

Can some one assist me in this?

Sure thing. For what it's worth, I think it entirely depends on how far your Chapter moved from its Progenitors. For a largely Codex-compliant Ultramarine successor Chapter, like the White Consuls, a new Curse is probably uncalled for, but I'd argue in favour of creating a separate Curse for the Mortifactors. I'd even scratch the separate curses of the Novamarines, Marnes Errant, Howling Griffons and Flesh Tearers (although in the last case it's debatable. If it were up to me, I'd just give them the same Black Rage as the Angels, but lower the Insanity requirements for them). So my point is you don't necessarily have to create a new Curse, and you can also pick the Codex Chapter Curse any time, as the slow descent into "We're so much better than everybody else" works everywhere.

If you want to have a separate curse, you either slightly modify existing ones, or combine elements of them. To suggest something more specific, it'd be nice if you could tell us a few key things about the Chapter (like the parent chapter; the nature of the divergence, if any; core elements of belief and culture).

That doesn't really help too much. Is there a chart or something I can use so that any schmuck can do it?

Sorry, I skipped stating that here's no custom Primarch's Curse chart in RoB's chapter creation rules (or anywhere else), and I didn't find homebrew Curse generators either - most likely because a Curse is the condensed and exaggerated expression of your Chapter's core beliefs and philosophies. There's a general stand-in Curse for Codex Chapters of unknown origin on p. 101 in Honour the Chapter, and if you know the Primarch, you can always fly with the Progenitor Chapter's Curse, but you have to design one yourself from scratch if you want a unique one, just like when choosing Chapter Trappings. I was thinking a bit further ahead and tried to offer ideas for that.