Okay, I am making an adventure for call of Cthulhu, a horror roleplaying game based on the works of Lovecraft. Here is my basic storyline so far
Settting: 1920s Massachusets, probably Arkham
Mobster hires investigators to find where a local black market fence went, as the fence owes the mobster money.
Investigators find fence dead with strange claw marks on him.
Mobster rehires them to find the killer, so he can get the money from him by any means.
Mobster calls them in late at night when his men shoot something crawling around outside of his house. It turns out to be a cultist.
Mobster remarks about a beautiful piece of stolen sculpture the fence gave him earlier, a day before he died.
What the players do not know is that the sculpture is a water idol the deep ones want. The cultist wanted the idol and came for it.
The players find out about a cultist meeting on a deserted beach and go to observe the cultists call the deep ones.
The mobster find out about it and is intrigued. He asks them to find out about these cultists more, with the promise of money. He is attacked some time en masse to get the idol. The deep one’s eventual goal is to flood the city by rain and take back ancestral ritual sites. They plan on calling in an old one, and need the idol for the inscriptions on it. I don’t know how to continue from this. They may track down where the idol came from.
Any ideas? Props or other things I could use, like newspaper clippinjgs or diaries? Ledgers?
Thanks!
Anyone who reads this will think me mad. Indeed, I myself question my own grasp on reality. The horrors I witnessed cannot possibly be described in mere words. But this is exactly how they looked… 
I’ve always loved the way Lovecraft said things could not possibly be described, and then went on to describe them.
How many sessions are you planning for that plot?
With any adventure design for an rpg, and especially with CoC, you have to take into account that “a plan is a list of things that don’t happen.” Investigators are always in way over their heads, and even relatively minor eldritch critters can wipe out the entire group with great speed, or render them so insane that they’re effectively out of play in any case.
Similarly, with luck and a good plan, a group can completely derail cultists as well, before they get the summoned-horror thing going. What happens on the beach if they decide not to just observe from good vantage points, but to use those good points for sniping? Hard for a ritual to carry on when cultists’ heads are being introduced to rifle slugs. They’re only going to observe because they’re not really cut out to do that kind of thing? (Which is a fine type of character to play CoC with–far more interesting than more violence-capable ones, in my view.) That would minimize the chances of the PCs wrecking the calling–but then why in the world would a mob boss hire them to deal with killers?
This kind of thing is why the thought of making anything longer than one or two sessions for a CoC game makes me gibber. I think it’s sort of like why Lovecraft wrote mostly short stories instead of longer, more involved ones–it’s simply very hard to maintain the right kind of atmosphere, with characters in anything long-term, and have suspension of disbelief maintained.
Call of Chtulhu… my favourite RPG (well… that and Paranoia) 
Newspaper clippings are a good start, printing them on newsprint makes them seem far more authentic. Diaries are more problematic – players invariably seem to want to read the whole thing rather than just the last entry (though you can get around this by having the book badly damaged with only a page or two able to be salvaged).
Try also old and weather-beaten maps; scraps torn from volumes that man should not have ken of; copies of old/period photographs (make the subjects NPCs); and small totems or symbols (perhaps found around the neck of a slain cultist/half deep-one).
Another good prop is puzzle pieces. I’ve used a small 3-D perspex puzzle (looked cystal-like) than made up into diamond. You can either give the pieces to the players over the course of several sessions, (each time they achieve a goal they find another piece), or (as I did on this occasion) put them is an urgent, time-pressure situation and then give them the whole thing. You should have see the scramble when I dropped the pieces in the middle of the group. 
Thanks for the advice!
I might prune it down a little.
Well, if the idol is important to Deep One plans, they will use divination magic to find out where it is, who has it, or both - so you don’t HAVE to figure out a mechanism for them tracking the idol, which can save you having to answer some player questions (‘How did they know we had Armitage hold the idol for safekeeping?’)
Have the Deep Ones replace an NPC that is important to the character’s plans at some point in the scenario, and reveal this at the worst possible time. This will get the players doubting what they have figured out so far. There is a spell in the Lesser Grimoire that allows the caster to take the form of someone and shift between forms at will, it’s normally cast by those ancient snake-people and requires you to completely consume the corpse of the person you are imitating, but since the spell is learnable by other species I’m sure the Deep Ones have access to it.
If you really want to get the party on edge, hand notes to specific players in-game (maybe after rolling some dice behind the screen) that say nothing more than ‘Don’t tell the other players what is written on this note’. The players may think that you are instructing the other players to behave as if they are mind-controlled or having them figure out something they might not want to share with the party.