Thanks for the reply.
Yes, I know that you need to create usable Ink in the tier 2 Alchemy lab. And I did see (and train) the 'Body Modifiactions' Loyalty upgrade with several Servants. My problem is that I simply can't find how and where the panel is to apply said Ink to a willing Servant. If my memory serves me right it used to be in a Servant's inventory screen, but I can't find it there anymore.