After the harrowing main story, this one is a bit of fluff, and a place to put all the money you've accumulated throughout the game.
Start by picking up your Aunt Lily at the designated marker, then driving her back to Sammy's. As ever, there's technically a time limit, but it's not severe. Inside, talk to Lily to find her optimistic about restoring the place- even with all that cultist blood still dripping down the walls.
Renovating the bar is mostly a matter of money- talk to Aunt Lil to see the various things you can improve, then drop the relevant amount of cash to see the renovations spring into being.
Sammy's doesn't offer much in the way of return-on-investment, so definitely save this task for the endgame, when you're taking in more cash than you count. Overall, renovations are going to set you back roughly $170,000.
As you continue to sharpen up the place, story missions will be triggered. The first is 'Angry White Boys.' A mob of Dixie Mafia will stir up trouble outside- exit through the garage and mow them down (this is another police-will-turn-a-blind-eye type of fight).
One of them- the chubbier guy in the driveway, is the leader, and can't be killed. Dispatch the rest, then have a chat with him, interrogation style.
Go back and talk to Lily to complete the mission.
The next story mission is 'Our New Bartender.' Talk to Lily, then talk to Reggie at the bar to get the deets on a pink slip race out in the Bayou. Luckily it's not the deep bayou. Follow your marker to Shorty and talk to him to begin the race
I liked the Traviata for this race- the speed is handy on the straightaways, though it can be easy to bounce right off the dunes. However, you can also pull this off with a slower car- most of this track is very narrow- as long as you can get ahead of Shorty on the first corner, you can block him from passing you for pretty much the rest of the race. Even if he pulls ahead, you get a massive advantage just before the finish line- Shorty will tend to get stuck in that last 90 degree turn.
Winning will trigger a cut scene with Reggie, mission completion, and unlocking the Harrier for your own personal use. It's a sweet ride, doesn't corner quite so badly as Shorty makes it seem, and there's an achievement for driving it around long enough. Start by driving it back to Sammy's and continuing the repairs.
The final story mission outside the bar is 'Lil' Ernie.' This one takes you to disreputable dope den by a lighthouse.
It's possible to slip in through the back- there's a back door and open window you can climb through. But it's probably easiest just to whistlekill the lot of them from the front door.
When the first floor is clear, head upstairs to finish the job. There should be three guys left- one of them the Dixie leader you chatted with earlier. Interrogate him again to bid a more permanent farewell to the rotter.
Follow your marker outside, and to some massive cellar doors.
Don't worry about an ambush- there isn't any. Just cut Ernie loose to complete the mission, then head back to Sammy's.
Finish renovating the bar to unlock a little epilogue- 'Openin' the Doors'.
Talk to Lily to launch a proper grand opening. You'll automatically suit up downstairs. You can mingle and chat with folks as you make your way to the bar, including Father James.
Once there, you and Lily will make some speeches, raise a toast to Sammy, and close out the last of the DLC