The Warble of a Smitten Knight - Toussaint (Blood and Wine)
This quest becomes available during the The Beast of Toussaint Blood and Wine Story Quest; it'll automatically be added to your quest log. If you do not complete this quest before meeting Captain de la Tour as part of the Capture the Castle Quest then this quest will automatically fail.
For the first part of this quest you'll need to head over to the medic tent near the arena that you faught the Shaelmaar in during The Beast of Toussaint. Here you'll find Guillaume who is the quest giver. After the first part of dialogue for this quest Guillaume will ask you fi you wish to participate in the practice rounds - you should do this so that you know what to expect for the tournament.
If you refuse or once you've completed the practice rounds you'll now have to go to the Scribe who gives out the crests which you join the Tourney under. You'll have two options for this part, you can join as Geralt of Rivia or Sir Ravix of Fourhorn. Important if you plan to get the Virtue of Honor for the There Can Be Only One Side Quest then you must join the tourney as Geralt of Rivia.
Tip: Before participating in any aspect of the tournament it's recommended you save your game - incase you make an oopsie. As a whole the tournament isn't very difficult though, so you shouldn't have too much of a problem winning.
When you're ready to begin approach the shooting range (marked on your compass/map) and a scene will automatically begin when you get close. The man here will offer to pay Geralt in a round of Gwent which, if won, will make the crossbow shooting round easier.
Shooting Tournament: Not much advice I can give, when I won the Gwent game before the match my opponent only got down two targets before I got down all of mine - making it trivial
Guillaume will take you to Lady Vivienne's tent between tournament rounds, there all you have to do is inspect items in the tent using your Witcher Senses until the next round of the tournament begins.
Racing Tournament: This is the hardest of all the challenges to win because the controls for mounted combat aren't the best. To slow down time and make it easier to hit your target hold down I highly recommend you stop sprinting on Roach long before you get near the target - only use Canter when you get close enough to one of the targets. For the most part throughout this tourament it'll alternate between a ground target and a crossbow target; around Checkpoint 7/9 this will change and you'll get some double ground targets in a row.
After you complete the racing tournament you'll have a scene at a dinner table and one of two knights will confront you. If you chose Geralt of Rivia when entering the tournament then it'll be Anseis who confronts you and challenge you to a duel, if you beat him in the mounted duel then you'll receive the Virtue of Honor for the There Can Be Only One Quest. Note: Aside from this mounted duel there is only one other way to obtain this virtue.
When the mounted duel is over it'll be night time and you'll be tasked with finding Lady Vivienne. Enter her tent and then go out the back to update your quest, you'll notice the same bird from an earlier cutscene that will take flight and chirp. Use your Witcher Senses to follow the chirping (as pictured below).
Along the way while following the bird you'll have to fight a few Panthers, use Beast Oil if you'd like to make the fights much easier. Further up the path you'll encounter a Wight inside of a cave, Necrophage Oil and Axii is what it is weak against. Once you're through the cave it'll be a short distance until you get a scene with Vivienne.
She will tell you what curse ails her after which you'll have to return to the tournament grounds (or fast travel there) and tell Guillaume about the curse or not. I recommend you tell him as it'll open up an additional way of curing Vivienne, it'll also cut out a lot of travel time for the quest later.
During your conversation with Guillaume you'll have to decide if you wish to finish the tournament or not. If you wish to get the Virtue of Valor for the There Can Be Only One Quest you must finish the tournament. This is your only opportunity to obtain this virtue.
Depending on if you told Guillaume about the curse or not will change the last few parts of this quest. If you did tell him and agreed to transfer the curse to him then he'll accompany you and Vivienne to the pond in the woods and your quest will automatically complete here. If you refuse to tell him or refused his method then you'll company Vivienne to the pond and you'll have to return to the tournament grounds to turn in this quest.