So I was a little late to discover the sylesti brewing part of the fall festival, but that asside, when exactly can I add my additives? I bought two vampire dust with my chocolate coins and I'm afraid I won't be able to use them. Did I screw up and it turns out I was supposed to add them before brewing r something?

unless you have an expedient brew agent, you almost definitely won't be able to complete any newly-made brews at this point, since it takes a long time (and cutoff date is the 15th, at which point all unfinished brews will vanish)

however, for future reference:

You need to purchase a cauldron (or pot, or sandbox, or... whatever is going to happen with winterfest this year).
There are three 'permanent' ones- they only cost 5k gold to use- temporary ones can also be purchased in addition to these for 75 diamonds/ 150000 gold/ 300 tokens.

Once you have purchased that, you need a catalyst of the theme you want (eg, [Qitari] Haunting Mists or [Luffox] Flying Squirrel)

Click the cauldron (or [insert season applicable device]), select the catalyst from the drop down menu.

Once you've placed the catalyst in, clicking the brew (or other applicable) brings up another popup, and from there you can toggle between the two additive pages.

[Calming Powder]/[Reactive Crystals]/[Herbs and Spices] (different in other fests) are used to maintain a brew's condition. Once every ~35 minutes brews tick over and want a specific one of the three, and adding it a) adds to the completion %, b) raises the quality (if they've had an expedient used on them, though, the cooldown between is removed, allowing for the brew to be swiftly completed)

Vampire Dust can only be added to your brew if it has had a blood vial used on it, as vampire dust adds a hidden trait's genetics under a visible trait (unless the brew is a Majestic theme, which cannot have hidden traits)
(2 vampire dust = 1 hidden, so you can only use 2 max on a brew with a lesser blood (a 1v brew), a max of 4 on a greater-blooded brew (a 2v brew) or max 6 on an exceptional (3v))

Thank you! It isn't really explained very well anywhere as far as I know. It's not a new brew it's one I started just over a week ago, it's close to being done actually.

it's explained in the fest overview fairly succinctly