I get a lot of questions from readers, and most of the time, the answers are fairly short. When I feel the question or the reply would be valuable to others as well, I make a post with a collection of them and post them in one go. Today is one of those posts.

"How would ancestral rituals be conducted, would Hermes Chthonios be invoked before and after an ancestral ritual and when could I hold sacrifices to my ancestors?"
Ancestral rituals? To honour the deceased like on Agathós Daímōn? The ancient Hellenes believed that the moment a person died, their psyche--spirit--left the body in a puff or like a breath of wind. Proper burial was incredibly important to the ancient Hellenes, and to not give a loved one a fully ritualized funeral was unthinkable. It was, however, used as punishment of dead enemies, but only rarely. Funerary rites were performed solely to get the deceased into the afterlife, and everyone who passed away was prepared for burial according to time-honoured rituals.
During the actual funeral, a related mourner first dedicated a lock of hair, then provided the deceased with offerings of honey, milk, water, wine, perfumes, and oils mixed in varying amounts. Any libation was a khoe; a libation given in its entirety to the deceased. None was had by the mourners. A prayer to the Theoi--most likely Hermes Khthonios--then followed these libations. It was also possible to make a haimacouria before the wine was poured. In a haimacouria, a black ram or black bull is slain and the blood is offered to the deceased. This blood sacrifice, however, was probably used only when they were sacrificing in honour of a number of men, or for someone incredibly important. Then came the enagismata, which were offerings to the dead that included milk, honey, water, wine, celery, pelanon--a mixture of meal, honey, and oil--and kollyba--the first fruits of the crops and dried fresh fruits.
Unlike the ancient Egyptians, the ancient Hellenes placed very few objects in the grave, but monumental earth mounds, rectangular built tombs, and elaborate marble stelai and statues were often erected to mark the grave and to ensure that the deceased would not be forgotten. Grave gifts were allowed in many places, but could not cost more than a set amount all together. These elaborate burial places served as a place for the family members to visit the grave with offerings that included small cakes and libations. The goal  was to never be forgotten; if the dead was remembered always, and fed with libations and other offerings, their spirit would stay 'alive' forever. That said, especially in Athens, names on grave markers were restricted to women who died in childbirth and men who died in battle.
This is also how we can honour the deceased today. Especially on Agathós Daímōn, we can pour them libations of unmixed wine and small cakes. We can remember how they changed our lives and helped shape us. We can remember their deeds. This way, they will never be forgotten. Hermes Khthonios was invoked during funerary rites because he brought the souls of the deceased down to Haides, but when remembering tour loved ones, He needs not be invoked.

"I've seen your meditations posts and they are so beautiful. I know you said you would like to have an oracle and I agree. But my question is: what do you think about dreams and the Theoi sending messages through them?"

I completely believe and trust in UPG (unverified personal gnosis), amongst which dreams and meditations. I truly believe the Gods send them and that they should be listened to. That said, I have the same issue with UPG as I have with modern oracles: we haven't been trained to listen and interpret these messages. Ancient oracles lived and breathed the voices of the Gods, that was their purpose. As much as we like to think we understand everything the Gods try to tell us, often only time will tell if we got it right--and more often than not, the message was far more subtle than we thought it was. So while I think dreams and oracles have a lot of value, I also think we need to be very careful in interpreting their messages.

"If you make a vow to the Theoi or promise them a certain votice offering, but you - for some reason- are not able to follow through on that promise, would making an appeasment sacrifice (different from the original promised offering) to make things right?"

That... depends. It depends on why you couldn't give the promised offering and why you promised that specific offering in the first place if it is--appearently-difficult to acquire and give. If there are circumstances outside of your control that totally and completely prevent you from performing the sacrifice (It's a unique item no longer in your possession, you'd have to break the law to perform the sacrifice at the spot you promised, etc.) then a placating sacrifice with an equal votive offering would be alright, I suppose, but I would warn against making grand promises in the future. In essence, if you use your 'get out of jail free'-card, it's gone the next time you'd need it.