Completing the Found Witch Coven decision gives all these goodies:

• Health: Medium Boost
  
• Fertility: +10%
  
• Hostile Scheme Resistance: +10
  
• Hostile Scheme Power: +10
  
• Host Grand Rite activity every 5 years (where you can learn new skill traits, or gain a stat boost, or just blow off some stress)

You can't go find your own witchyness. You must wait until it comes to you.

Certain rare events pop up with a chance of giving you the trait. Someone might offer to convert you, but it's more common to come across an obscure book to study (in exhaustive detail).

It's rumoured these events are more likely with an active Learning focus, or maybe just having a high Learning score. Or perhaps once a thrice brinded cat hath mew’d (says Shakespeare).

'Found a Witch Coven' is available under Major Decisions. But first you'll need all this:

1. You have the witch secret / trait.
   
2. You are your House head
   
3. At least 7 house members have the Witch trait or secret
   
4. At least 60% of house members have the Witch trait or secret known by you
   
5. Your Player heir has the Witch trait or secret known by you

Naturally you'll need to convert a bunch of your House members. Convert to Witchcraft is a personal scheme, and usually succeeds. Try to form your coven early, and dodge the need to make a huge number of conversions to get the 60%. And select the option to reveal yourself to each convert, or else no one will know to attend your Grand Rite and it will be a wash out.

Any kids you're directly educating are easily corrupted, since you gain a guaranteed conversion option shortly before they come of age.

Thankfully you're not limited to converting House Members, so you bring in other talent too. Those with Gardener, Herbalist or Mystic traits are great, as they might teach you these skills.

Now you can host a Grand Rite as an Activity every 5 years. At the party you usually get a choice between a skill stat increase, or even learn a new skill trait from someone attending. Or else you can just dance like a lunatic for a massive stress loss.

But your own Grand Rites are not the only party in town. Landed coven members can host their own Rites (well, certainly Dukes can). Attend these to get more of the same skill bonuses. For this reason you should convert all your major vassals to the dark and ancient ways.