The Landsmeet is proposed by Arl Eamon of Redcliffe as a way to deal with Loghain's treachery, by putting him up against a vote of his peers.
Upon arriving in Denerim for the Landsmeet you are informed by Queen Anora's handmaid, that the Queen is being held captive by Arl Howe in his estate in Denerim. She requests your assistance in freeing her.
Arl of Denerim Estate
This Estate is far more than just the place where Anora is held captive. It is in fact the only place to gain supporters against Loghain (outside of the Alienage) and is therefore the only thing that will ensure you win the Landsmeet. Your chances of winning the Landsmeet are begun in positive and negative support from the minute you arrive in Denerim and how you structure your quest to Arl Eamon's request to look around and see if you can garner support. The first place you need to go is Arl Howe's Estate.
You have to sneak into the Arl's estate by procuring guard uniforms (which you will be automatically changed into once you have them). Note: You cannot have Shale in your active party during this quest. If you do, before you move to sneaking into the Estate, Anora's handmaiden will request Shale be replaced. Once you reach Anora's door, you will find that a mage has put up a forcefield on the door. You need to find the mage, who Anora says will likely be with Arl Howe, to disengage the forcefield.
As you make your way through the Arl's dungeon, you will come across many people imprisoned there. Rexel, the missing soldier for the Chanter's Board quest is down here. Two other notables are also here, both relatives to Banns attending the Landsmeet. One is tied to a rack in a seperate torture chamber and the other behind Arl Howe. Once you release and speak to them, their family will be waiting at the Gnawed Noble Tavern. The former Arl of Denerim's son is also locked up but, providing you have released the other two, his help isn't necessary. If you tell him that your rescue will be very costly, he will give you a key to a chest containing 40 sovereigns. With a high enough Coercion, you can take the key and let him rot. You should however release him as you might need his father' support to win the Landsmeet. You get a very high positive in the Landsmeet for releasing him.
Once you finally find Howe and take him down, return upstairs to Anora's room. Anora's response isn't one you want to hear when you're about to decide who to give the crown to. As you make your way to the frontgate, you will be accosted by Ser Cauthrien and a large number of Loghain's guards. You can either surrender or put up a fight. While it is technically possible to win the fight, it is not worth the effort and surrendering will be much quicker. If you surrender or fall in battle you, along with Alistair (If he is with you), will be taken to Fort Drakon.
Note: If your lockpick skill is high enough, or another rogue in the party has a high lockpick skill, there is a really nice Kaddis and collar for Dog being kept in the Dungeons in a locked chest in the warhound room.
Battle with Ser Cauthrien and Loghain's Guards
If you're determined to go through with this fight, pull back as soon as you can into a corridor or room to force the archers to chase you down, gather together and provide an easier target for mass crowd control. If you retreat fast enough and far enough back into the corridor you came from and into the first room on the right (further down the hall and opposite the side from Anora's cell), it is possible to lure Ser Cauthrien by herself into the room, where she can be dealt with easily. Her remaining guards are much easier to handle once she is out of the way, and they can be taken care of at your leisure.
Should you fall or surrender, see Fort Drakon for information on the release/escape.
After Fort Drakon
Once you have escaped or been released from Fort Drakon, you have the chance to make good on getting people to side against Loghain. Everyone you need to find who has had a son/brother imprisoned, can be found at the Gnawed Nobel Tavern. Give the sister the ring if you want her support. If you do not give her the ring, you will lose her support at the Landsmeet. There are 4 individuals who will give their say at the Landsmeet, one of them will always vote for Loghain, you need to procure the support of the other 3.
When you do the quest at the Alienage, make sure you pick up the 'Slave Papers' and then show them to Arl Eamon.
The Landsmeet Decision
It is possible to win the Landsmeet, but you cannot use evidence or expect support if you have not followed through. If Arl Eamon was not shown the Slave Papers, you cannot use this in your defense. At no point during your negotiations should you ever say anything about "Having left King Cailian to die". This will gain you negative approval points in the Landsmeet. If you have followed through, gained support and set out your points according to those accusations you can support, you will win the Landsmeet. Again though, expect Anora to turn on you...big surprise.
Once the Landsmeet decision has been reached, regardless of whether or not you are victorious, a fight will break out with Loghain. It will eventually be stopped and a duel will be suggested. You can choose anyone to be the Champion, though the most satisfied at the prospect will be Alistair.
If you do not choose Alistair, you will be given the chance to show Loghain mercy with interesting consequences. No spoiler given.
It is worth noting that if you obtain enough support from the nobels-- including getting Anora to denounce her father-- you will be declared the winner of the Landsmeet and will not have to engage in the mass combat against Loghain's forces. You will still need to duel him, however.
The Heir to the Throne
You can either attempt to persuade Anora and Alistair to marry (provided if Loghain was killed you were the killer and not Alistair as Anora will refuse to marry the individual who killed her father), put Anora on the throne or put Alistair on the throne. Alistair may be upset, but you will not lose many points with him. If you are a human noble and female, you can propose that you and Alistair rule the throne together. If you should spare Anora being executed, she will admit that she would not have shown such mercy to Alistair.
It should be noted that if you are romantically involved with Alistair and not a human noble, he breaks up with you, telling you that he needs to have a child and that you cannot be his queen.