Please, whenever possible, link this post to help out your fellow players.
This is best with 2 players, if possible, to maximize profits.
You'll need an Arcade, here's a list of all of them.
I prefer the Eight Bit Arcade because of its location and accessibility to the Garage, as well as the fact that it makes most preps spawn in the same locations for easier completion of preps.

Here's a list if guides that'll help with getting started with the Diamond Casino Heist:

Scoping Out

Scope Out Guide [Skip to 4:36]
All Access Points
All Points Of Interest / POI

Tip 1:

While doing Casino Heist preps, you can make Lester shut up and show the mission objective faster by giving one of your contacts (Mechanic, Assistant, hell even Brucie) a call, as long as you know the contact you call will pick up the phone and not just leave you ringing.

Tip 2:

[Make sure to call Simeon or Martin and request a job before you start a prep, as you cannot request a job while doing Casino Preps]
In some missions you are unable to call Lester and call the cops off of you. During these missions, you can get the cops off of you for free by calling Simeon or Martin and request a job. Once you get the job, join it.
AS SOON AS YOU JOIN THE JOB, LEAVE AS SOON AS POSSIBLE. If you don't quit, you run the risk of the Host starting the job and therefore cancelling the heist prep.
Once you leave the job, you'll be back in Freemode with no cops on you, and you'll still have your heist prep equipment.

Note:

There are two Access Points that are required for the way I run the heist, in Aggressive and The Big Con (Gruupe Sechs), being the sewer entrance on the right side wall in the first opening, and the tunnel entrance leading into the center of the track.

Scope out the Casino

Look for Artwork and Gold, as they are the best (unless Diamonds are available in the vault. In that case, Diamonds are the best)

Changing vault contents

If you've completed all three approaches (Silent & Sneaky, Aggressive & The Big Con), you'll have unlocked the ability to cancel the Casino Heist by calling Lester. You can use this to cancel the heist if you get Cash, or any Vault Content item you don't want.
If you find the Vault contents and get something you don't want, like Cash, you can call Lester while still in the Casino and cancel the heist. Go back to the Arcade and pay the 25k to start the heist again, and you should have a different item for your Vault Contents.
This will essentially allow you to get a second Vault Content item by trading 25k to start the heist a second time with about 8 minutes of finding and hacking / killing the Casino Security guy and going to the Casino.
If, once again, you get a Vault Content item you don't want, exit the arcade, call Lester & cancel the heist, enter your Arcade, rinse and repeat.

Personally, I cycle between Aggressive and The Big Con (Gruupe Sechs).

Now, because I prefer doing the heist through the Aggressive and The Big Con (Gruupe Sechs) approaches, I have little to no experience doing the Silent & Sneaky approach, and thus will be basing this post off of my personal experience in the other two approaches.
Note I'm not going to go into every single prep mission, mandatory or optional as that would take too much time for me. You can look up the rest of the preps on YouTube or Google. I'm only going over my preferred Gunman, Driver, Hacker, the preps that I prefer to do / prefer not to complete, and how I complete the heist.

Aggressive

Gunman: I prefer Patrick MacReary, 8%. I can't remember the other weapon available, but I know he allows you to choose an LMG with 100 round mags. I prefer to use the LMG. You can unlock by finding him in the back of a police van. Take the van to the objective area by following the yellow marker and you'll unlock him.
Driver: I prefer Karim Denz, 5%, which has decent vehicles. I prefer the Sentinel Classic.
Hacker: I prefer Paige Harris, 9%, which you can unlock by owning a Terrorbyte (which you can unlock by owning a Nightclub).

The preps I prefer to do are:

Unmarked Weapons, Getaway Vehicles, Hacking Device, Thermal Charges, Vault Explosives (I prefer to use a Stromberg while doing this mission), Reinforced Armor, Vault Keycards, Patrol Routes, Duggan Shipments, Security Intel (unlock by owning a penthouse and completing the penthouse missions), and Security Pass.
For the Reinforced Armor, I only do this if I have Gold or Diamonds as it slows you down. I also only do this if I am a low level or if I have low level teammates, because low level = low health. If I have Artwork or Cash I don't do this.
As for the Boring Machine prep, you don't need to have the Boring Machine to access the sewer. All you need to do is have took a picture of it and sent it to Lester during the Scope Out prep.

The Big Con

Gunman: I prefer Karl Abolaji, 5%. He has access to the Mini Uzi (I forget the in-game name) and the double barrel shotgun. I prefer the Mini Uzi. *You're going to be mostly going up to people and meleeing them / knocking them out instead of shooting them, unless you get caught, then you'll be shooting them.
Driver: I prefer Karim Denz, 5%, which has decent vehicles. I prefer the Sentinel Classic.
Hacker: I prefer Paige Harris, 9%, which you can unlock by owning a Terrorbyte (which you can unlock by owning a Nightclub).

The preps I prefer to do are:

Unmarked Weapons, Getaway vehicles, Hacking Device, Entry Disguises (2 preps), Vault Drills, Exit Disguise, Vault Keycards, Patrol Routes, Security Intel, and Security Pass.
I don't do Duggan Shipments as usually use my weapon to melee / knock out the security guards at the Casino, and since a melee is a one hit one kill, regardless of level of armor they're wearing, I don't usually do this mission.
Completing the Yung Ancestor missions allow you to have the Grotti Furia, a supercar, as an extra getaway vehicle, although it spawns in the parking lot which is surrounded by cops. I would not recommend doing this.
Again, 2 players are recommended. Start the Heist, get to the Vault. One of you work on the first main room with the big circle table in the centre while the other one will start hacking whichever doors lead to 2 or 3 Artwork or Gold / Diamond / Cash tables. Always go for the rooms that have more than one cart.
Tip: With Gold / Diamonds, you cannot sprint / run, you can however trot. If you go into First Person and tap / press and hold whatever your sprint / run button is, you'll see your character's weapon / screen shake, indicating that you're trotting. Doing this in First Person is actually faster than Third Person.

There are two was I like to lose the cops

1, through the sewer, and 2, by helicopter (Helicopter Route)
[Disclaimer: This strategy is not recommended if you're trying to get the Elite Challenge, as this will most likely take more than 15 minutes, or however long the given time is.]
Once you are out of the Casino, make it to the road. Drive to the sewer opening by the Casino, and if you don't have a car, just run to said sewer opening.

Note:

If you go back the way you came, or anywhere close to the Casino after you lose the cops, you will gain a wanted level again.
Proceed through the tunnel. You'll find a metal gate that you have to pass under, a small vent on the right side, and another vent on the left.
You'll come across 2 paths. Go up the metal ramp on the left. Proceed all the way down the tunnel and down the escalators and ramp downwards. Cross diagonally over the 2 train tracks into the next tunnel with a dirt floor.
You'll find 2 tunnels, left and right. Take either one. You'll find construction, go straight. After this, you'll see a big room. Go straight, take the immediate right and go up the ramps leading to the opening to the highway. If you dont have a car, get one or two and drive to the Buyer.
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Links to my other posts, which consist of guides, and other useful things I've found & posted.