This is The Obvious Approach version of The Big Score mission.
You will require 3 heist setup missions:
  • Driller
  • Sidetracked
  • Getaway Vehicle.

The Driller Mission:

The 'Driller Mission' will be displayed on the map with the letters Hs.

Head to the La Mesa area.

You will be prompted to steal the Driller.

You can do this 2 ways:

  • With stealth and no cops.
  • Go in fast, and get a wanted level.

If you choose stealth, sneak through the back alley on the east side of the truck.

Stay in stealth mode all the way to the cab, then hop in and drive away.

If you did it right, you will have no wanted level.

If not, lose the cops. Head for the freeway and keep driving until you're clear.

Once you're clear, drive the truck to the way point near the bank.

To get gold:

  • Sneak Thief -Steal the driller without being detected. Do the stealth method, with no wanted level.
  • Time -Complete the mission within 5 minutes. Don't get a wanted level and you should have lots of time to make it to the way point.

The Getaway Vehicle:

Just like previous Heists, you need a getaway vehicle.

Steal a vehicle that can seat four.

Once you find one, Call Lester, he'll want it parked in the bank's underground parking area.

You now have the chance to customize the vehicle. Take it to the nearest customs shop and add bullet-proof tires, armor and some speed improvements.

Now head to the way point and park it.

The Sidetracked Mission:

The 'Sidetracked mission' will be displayed on the map with the letters Hs.

Head to the railroad yard southeast of Sandy Shores.

Two targets will appear on the map, take them out. Snipe the two workers from a distance to avoid detection.

Now go up the stairs to the control room, walk up to the controls to flip the railroad switch.

Once the train arrives, switch to Trevor.

Fly the chopper to the train yard and locate the train engine.

Once the engine is located, position yourself over it. Use the rudder to make small adjustments to your position.

You will be prompted to hit the right directional button. It will activate an electromagnet and the engine will be pulled up to the chopper.

Now head back to the airfield.

Get yourself hovering over the flat bed. When prompted, hit the right directional button. It will deactivate the electromagnet and the engine will drop right in place. Repeat this for the flatbed train car.

To get gold:

  • Time - Complete the mission within 5 minutes and 30 seconds. You'll likely require practice and a steady hand. Try not to waste time with small adjustments. Drop into position fast, magnetize, then deliver.
  • Undetected - Steal the train without being detected. Snipe the two railway employees from a distance.

The Big Score - Obvious Approach:
( For The Subtle Approach version: GTA 5 Walkthrough: Mission 76 - The Big Score (Subtle Approach) and Prep )

Head to the Vanilla Unicorn as one of the three characters and watch the cut scene.

Now jump in the car and drive to the bank.

Once there you will be automatically switched to Franklin.

Drive the driller forward, turn right and drill through the wall. Turn on the cutter by pressing the right directional button.

Once through, reverse and park the cutter. Run into the vault and plant Sticky Bombs on the doors. Now leave the vault and blow them open.

Now take cover on the right. Protect your gunman from the incoming NOOSE teams.

Once the right is clear, take cover to the left and fire on the NOOSE teams there.

Once clear, watch the cut scene.

A NOOSE team will begin coming through the vault. Run to the drilled hole, take cover to the side and take them down.

3 NOOSE guys will come down from the hole above you. Quickly take them out.

Once clear, switch to Trevor.

Now fly away from the city.

Once out, switch to Michael.

Now move forward to the bridge, doing the usual, cover, shoot, and move when clear.

When a chopper appears, use Michael's special ability, target the pilot and take it down before it does any damage.

Once at the bridge, you will meet the other crew members.

Just keep following the crew, doing the usual, cover, shoot, and move when clear.

Once you cross the bridge grab the med kit on the right.

Eventually, you'll find yourself near a large sculpture. Quickly take out the chopper, using Michael. Then clear the near by targets.

Now switch to Franklin and take out the remaining targets.

Switch back to Michael and take out the chopper.

Now keep following the crew and head down the stairs. Try not to get lost.

Do the usual, cover, shoot, and move when clear.

Once you're to the parking area, use your grenades and take out the police vehicles. This will help kill most of the targets. Now just clean up the remaining cops.

Two more police vehicles will appear, take them out.

Now get into the getaway vehicle and lose the cops. If the cops don't see your vehicle, it's much easier to getaway. Try to find a good place to hide. If your map is flashing red, drive.

Once you lose the cops you will be switched to Trevor.

You now need to destroy 3 Merryweather helicopters.

Listen to what Lester says. Get close and hold still when he says to. He should hit the Merryweather chopper almost every time.

Once clear of Merryweather, drop the gold onto the back of the train.

Come in low, and hit the "right directional button" immediately when prompted. A cut scene finishes the job.

Now land the helicopter and the mission ends.

To get gold:

  • Headshots -Kill 20 enemies with headshots. So many opportunities. Again, use auto aim, push up, and then shoot.
  • Time - Complete the mission within 16 minutes. Lose the cops quickly, and you should have this.
  • Cha-Ching - Drop the gold onto the train within 30 seconds. Don't hesitate, swoop down and drop the gold.
  • Accuracy - Finish with a shooting accuracy of at least 60%. Don't shoot wildly, make sure you're hitting a target, and use Auto aim.