bayoread:

• Khthonic deities are of the earth or ‘under’ it so offerings were made outside on low altars (escharai) or into specially dug pits.
• For libations wine was replaced with water, milk, honey or blood.
• Animals sacrificed were black in contrast to white, which were reserved for Ouranic (sky) deities.
• Rituals were traditionally performed at night.
• Seeing as animal sacrifice is frowned upon nowadays and not feasible for most practitioners, votive offerings can be offered instead, i.e. black animal figurines, predominantly sheep and rams.
• Other offerings may include coins as traditionally an obol or oboloi (plural) was used as currency in Hades.
• Epithets were used more than plain names.  Persephone  – Kore (Maiden,) Carpophorus (Bringer of Fruit), Despoina (Mistress), Praxidice (Exacter of Justice). Hades  –  Aidoneos (The Hidden), Necron Soter (Saviour of the Dead), Eubouleus (Good Counselor), Necrodegmon (Receiver of the Dead), Plouton (Wealth), Polydektes (Receiver of Many Guests), Theon Khthonios (God of the Underworld).
• Any food offerings were wholly burned or buried (holocaust), none of it was shared with the ritual attendees as with Ouranic offerings.
• Gods associated with or make the Underworld their home includes Hades, Kore, Hekate, Hermes Khthonios, Kerberus, Erinyes (Furies), Kronos, Thanatos, Charon, Hypnos, The Moirae and Nyx, among a myriad of others.
• Heroes and ancestors were also worshipped in this manner, thought to exert great power from their tombs and the Underworld. F’ex, Asklepios and Herakles.
• Caves or deep chasms were also thought to be gateways to the Underworld though mortals were discouraged from venturing there before their time.

Disclaimer – This is solely from personal research and in no way indicative of how everyone should approach Khthonic ritual. Feel free to omit or adjust anything you see fit.