Witch dedication

[Atalius]

How would this work?

Do I gain a basic lesson at level 2?

[Atalius]

Oh I see there's no such thing yet

[Sibelius Eos Owm]

It would be reasonable to assume a Witch Dedication would function identically to the pattern of most spellcaster multiclass archetypes. Of particular note here is Sorcerer, which also chooses its spell list based on another class feature which is not gained in the Dedication feat.

With the way Witch traditions are presented, with few other features riding on your choice of tradition aside from your First Lesson, it would make sense for your Dedication simply to ask you to choose a tradition for your patron without gaining any Lessons until later in the multiclass tree.

[Bandw2]

you'd probably get a familiar...

[The Ronyon]

A familiar from a single dedication feat would probably to damned good.
I don't like it, but it takes two feats to get a familiar through other dedications.

[shroudb]

I would assume that the actual benefits of your first lesson (apart from picking a spell list) like the Hex would be a level 4 feat.

Apart from that, regular spellcasting archetype.

The only issue really is the spellbook being the familiar.

I could see a few ways to deal with that: familiar instead of getting ANY skills (so 1 feat over 2 trained skills compared to other spellcasters), familiar over the actual cantrips (which you would have to buy at 4 with something like "minor spellcasting"), or something along those lines.