Both Epona and Cernnunos have been considered Pan-Celtic gods, which means that worship of gods of their sort are found across the continent where Gauls were. Epona is a Greco-Celtic goddess. The one line I could find about her (versus an account of worship of her) is Italian. It says she is a goddess of horses birthed of the pairing of a man and a mare. I find this somewhat spurious, being that it sounds way more Greco/Roman than Celtic, but it could be accurate. Celts were known for their animal gods, but the insular Celtic stories of animal gods have the gods turning into animals or being turned into animals or serving in the place of an animal. However, there is known to be a ritual kingship pairing of the king with the horse as a symbol of the land providing sovereignty to the king and so Epona could be a manifestation of that granted power.
Likewise little is known of Cernunnos, but he is also considered a pan-Celtic god because there are some representations of an antlered god found, many carvings of stags, and a few statues of gods with holes where horns could be put in. There are also a few inscriptions with different spellings to Cernnunos.
For both Epona and Cernunnos there are similar but not exact matches in Brythonic and Gaelic myths (the insular Celts), stories taken down by Christian writers about their pagan past. In the Welsh Mabinogion there are stories of Rhiannon, who rides a white horse and is forced to serve as a horse in the underworld for a crime she is later proven to be innocent (of killing her infant son). And in Red Branch cycles of Irish legend Macha is known for being able to run as fast as a horse and is forced by her husband into a horse race despite being heavy with twins, and therefore she stillbirths her infants. The similarities are striking, the dead infants and the mournful mothers having to serve in the place of a horse.
Also in the Mabinogion, Anwynn is an underworld god associated with stags and stag hunting. In Irish legends, Fionn McCumhail and his son Ossian are both associated with deer. Fionn falls in love with a fairy who is shaped shifted into a doe, and is said to not have died, but gone into Faery in the form of a Stag in one version of his life story. When Fionn’s (deer) wife Sibh gives birth to their heroic son Ossian he is born with a little patch of soft fur on his forehead and is named for that fawn patch. Because there are actual stories about these Irish Stag gods and Horse goddesses.
I offer these other heroes and gods as alternatives, because Epona and Cernnunos have no stories about them. Yet there is more evidence of their worship than there is in the insular Celtic isles. Because the insular Celts do not seem to have written inscriptions and make statues and reliefs as much as the Continental Celts (although neither wrote much). (As always we are just limited by what has been dug up, written about (versus hidden in museum basements), and trickled down into general (pagan) literature. So examples of how to honor such gods are more extant in the archaeological record than in the myths I mention.
If you wish to begin working with Epona (or Macha or Rhiannon), I suggest putting white horse imagery onto an altar, send support for abused pregnant women/mothers or imprisoned women with children as an offering to Her, and read poetry, song or stories out loud to her in her honor. When you lay down in hopes of dream communications from her, have something horse related in your hand or under your pillow.
If you wish to begin working with Cernnunos (or Fionn, Ossian or Anwynn), use antler or stag imagery on your altar. Give offerings from your hunts, wildcrafting, or even money or golden torcs on your altar. And work on hedgecrossing, at liminal places where one might find deer. Sitting in the woods under a tree with your legs crossed sending your soul to the fae lands where these gods and heroes reside.