A Familiar Place
So you're back in the Mansion. With the newly-found Helmet Key, you can finally unlock some doors that were inaccessible before, but there's a new enemy lurking around the Mansion: the Hunter. These enemies are faster, deadlier, and powerful than those simple zombies. It is recommended that you only use the Shotgun (or Bazooka for Jill) and the Colt Python (which Director's Cut players already have, but other versions will find this gun soon). The point of coming back here is to find the Battery, which will power the disabled elevator in the Courtyard, which grants you access to the next part of the game, the Underground.
When you're back in the shed, leave the room through the single door on the opposite side and then open the opposite door in the passageway. Walk forward a little bit to trigger a cutscene in which the new enemy, the Hunter, follows your path and opens the door behind you... not good. Quickly shoot the Hunter once with your Shotgun, and as soon as it gets back up, shoot it again. Repeat this process until it's finally dead. When it is, open the single brown door next to the gray one. Walk up to the desk in front of you and turn on the lamp, which fills the area with light. Take the Magnum Rounds from the desk and the Doom Book Vol. 1 from the shelf next to you (Director's Cut and Rebirth players will instead find a MO Disk, which is optional, but is required for the best ending in the game).
Exit the room and then open the gray door next to you: run forward a little bit, kill the Hunter waiting around the corner, and go into the safe room underneath the staircase.
You'll notice that either Barry (for Jill) or Wesker (for Chris) left some helpful ammo and items for you. Jill players will find some Acid Rounds, Shotgun Shells, and a First Aid Spray, while Chris will find a Beretta Clip, Shotgun Shells, and a First Aid Spray. Open your inventory and check the Doom Book you found in the previous room: open the book in order to find a hidden Eagle Medal, which is needed to access to final area of the game, the Lab. For now, drop off the ammo and items you found before (unless you need to keep some ammo) except for the Helmet Key, one powerful weapon (either a Shotgun or Bazooka for Jill), a healing item, and some ammo for the weapon you take. You're about to enter a boss fight, so save the game if necessary, and then leave the room. Head up the staircase and kill the Hunter who has his back turned. Run down the area of the hallway he was guarding, turn the corner, and open the single blue door. Head to the other side of this room and open the red door with the Helmet Key, which brings you to a large room with a piano: walk up to the piano and investigate it (Rebirth players will have to turn a record slowly on a gramophone to solve the puzzle), causing the Yawn to come out of the fireplace.
The Yawn is now more powerful and can take more damage, so be prepared. Fire at it with your most powerful ammo (Acid Rounds for Jill are a must for this boss fight) and dodge its attacks as best you can. Try not to stay in one area and the Yawn should be dead (for good) in no time. Investigate the hole in the floor the Yawn created; Chris players can jump down right away, but Jill players will trigger a cutscene with Barry. Both players end up in the Mansion's basement, but Jill players have a decision to make: you can either wait for Barry to give you the rope (which leads to the good endings) or not wait for him and go to the next area (which leads to the bad ending). Regardless, check the tombstone on the other side and press the switch, revealing a hidden ladder (Rebirth players will have a simple puzzle in which they must hit each flashing button in a sequential order with their knife... careful not to stab yourself!).
Climb down the ladder and be prepared for the next couple of enemies: Director's Cut players have to fight Hunters while the other versions have to deal with simple zombies. Kill all of the enemies in the area and go into the small alcolve near the ladder. You'll find some Shotgun Shells the ground as well as a Flamethrower (Rebirth only). Head to the end of the hall and open the brown door. Clear this room of enemies and pick up the two Green Herbs if you'd like, then head through the brown door (Rebirth players will have to place the Flamethrower into the panel on the left to unlock it). You'll end up in a kitchen. Chris players can find a Small Key on the counter; other than that, just step into the elevator that's around the corner (be careful of the zombie on the ground).
Getting the Battery
In this hallway, Director's Cut players will have to deal with hunters while all other versions fight zombies. Clear the room, pick up the Green Herb in the alcove if you want to, and then open the double blue doors. Kill all the zombies in this room and unlock the drawer in the corner for some awesome Magnum Rounds as well as the Scrapbook in between the two northern book cases. You can go through the optional room in the corner by pulling the sword from the door (by sliding the stylus up and down in the Rebirth version) or simply pushing the shelf forwards (all other versions). Inside the room you can find a Beretta Clip and a pair of Ink Ribbons, as well as some crows if you try to investigate the window.
Open the opposite door in the corner of the library to enter an even smaller library. Push the button that's underneath the white statue to shine a source of light. Push the nearby statue to this area in order to reveal a hidden room. You'll find Eric's Letter in Rebirth mode and an MO Disk in all other versions (which is required for the best ending in the game). Exit this room and return back to the hallway with the elevator (Rebirth players will encounter a chimera, but the best thing to do is to just run away). Go through the single door that's closest to the elevator. You'll find two boxes of Shotgun Shells (for Chris) or some Explosive Rounds (for Jill) as well as the mighty Battery.
Out of the Mansion
Now that you have the Battery, you can finally get the heck out of this mansion for good. Before you take the elevator down, you can go through the door at the end of the hallway to find a Blue Herb and Green Herb near a pack of zombies. Go on the elevator and open the door in the corner of the kitchen. After the short FMV, you'll have to deal with a zombie: quickly take him down and head through the door he came from. Ascend the staircase and open the set of double doors.
If you'd like the Colt Python (for all versions except Director's Cut) or three containers of Magnum Rounds (for Director's Cut players), quickly head through the single brown door on the left. Walk slowly ahead, as a hunter will pop out from the T-junction, so quickly take him out. Kill the other one left in the room and open the second brown door on the right. Head down the hallway until you reach the staircase, killing any hunters a long the way. Quickly go to the left and open the single brown door. Once you're inside, take the Shotgun Shells and Magnum Rounds from the counter, as well as the Orders from the table in the middle of the room. Once you do that, push the step ladder up against the moose head display and then press the light switch near the door. Climb up the step ladder, take the Red Jewel from the socket, and proceed to the tiger statue room (it's the room inside the T-junction in the green hallway if you forgot). Place the Red Jewel inside the statue and take the Colt Python (for all versions except Director's Cut) or the three containers of Magnum Rounds (for Director's Cut). Exit the room and head through the door at the end of the hallway.
Head back into the Dining Room and then head inside the Main Hall. Go through the double blue doors on the opposite side and then through the single door in the corner. Don't stop running until you reach the end of the hallway, as some nasty spiders are waiting on the ceiling. Carefully proceed down the next hallway and through the double doors, as there is a lone hunter patrolling. Now go through the gray door and into the safe room underneath the staircase. Take at least one weapon and its set of ammunition, the Square Crank, and the Battery. Save if you want to and then leave the room.
We're going back to the courtyard, so head back into the previous hallway and go through the metal door in the corner. Open the door on the opposite side, and then open the set of double doors.