"I know you aren't supposed to kneel to the Ouranic Gods, but what about the Chthonic Gods? I've seen that their altars are meant to be ground/floor level so would we kneel but still raise our arms? Or how would that go really? Because I see them needing to be praised as well, but technically they are Underworld Gods and would be metaphorically below us, so by raising our arms would that be offensive to the Ouranic Gods?"

Khthonic (from Greek χθόνιος khthonios, 'in, under, or beneath the earth', from χθών khthōn 'earth') literally means 'subterranean'. It refers both to earth deities like Gaea as to underworld deities like Hades. earth deities usually received worship like the Ouranic Gods (although not always) while Underworld Gods received a different type of worship (although not always).

For these Khthonic Theoi, an offering pit--'bothros' (βόθρος) in Greek texts--was used. Bothroi were usually dug when the occasion called for it, and closed up afterwards. Khthonic Theoi received special nighttime offerings of black animals, unmixed wine and special libations of milk and honey. Animal sacrifice was always done in a holókaustos--a sacrifice where the entire animal was burned and none of the meat was saved for human consumptions.

Hómēros in the Odysseia writes Circe advising Odysseus how to perform a libation to the dead:

"Draw near then, as I bid you, hero, and dig a trench two feet square, then pour a libation all around to the dead, first of milk and honey, then of sweet wine, thirdly of water, sprinkled with white barley meal."

The ancient Hellenes shaped a 'negative' (part of a) ritual by a reversal of normal practices. As normal practices dictated the practitioner stand before an altar with their hands raised, it's quite logical that they performed ritual to the Khthonic deities on their knees, hands down on the soil, sometimes beating down on it to draw the attention of the deities residing below. Where women wore their hair up or covered for standard ritual, they wore their hair down in Khthonic ritual.

Worship of the Underworld Khthonic deities was reserved for special occasions--mostly vengeance, death, and purification surrounding both. If the Underworld Khthonic Gods were worshipped in a state festival, They were generally worshipped in their Ouranic epithets.