I have never seen a Buddhist food offering at an altar. Can someone explain how it's done?
In our house, we offer flowers, tea, water to Lord Buddha for morning and evening. We offer food mostly rice & curry or whatever we eat for lunch before we eat around 11.30AM—12PM. Simplest way to offer food for Buddha, Dhamma, Sanga is keeping the "food" in front of Buddha statue at home and then saying, → I offer this food for Buddha, Dhamma, Sanga.
But you can optimize it further + gain the biggest benefits along the way using a couple of ways.
Always prepare the meals without eating the food. If you take 1Kg of rice for lunch, then don't eat it before offering to Lord Buddha.
Always offer the food to Buddha, Dhamma, Sanga qualities not Buddha or a specific monk alone. When you offer the food to the Thunuruwana, it effectively means you're offering that food to all the Lord Buddhas, all the Dhamma they taught and every single Buddhist Monk in the Past, Present, Future of the whole of Buddha Sasana. Offering like this means you're offering food for infinite Thunuruwan, so the benefits are also infinite till you reach Nibbana.
Also chant following gatha, Adiwasetu No Bante Bojanan Parikappitan Anukampan Upadaya Pathiganhatu Muttama
Other than offering food to Thunuruwan in front of your house's Buddha statue, you can also invite Buddhist Monks to your house to offer food and more items you wish to offer. But be sure to offer whatever you offer to Thunuruwan and the Sangha — Monks you invited — without just thinking to offer them to only those monks you invited.
How you set the Offer Scope affects the benefits you gained. If it's just invited monks, it'll be smaller than setting the scope to the whole Buddha Sasana.
Other than offering food to Buddha Statue, Invited Monks, you can also give food to other living beings. However, the biggest benefits come only when you offer food to Buddha Sasana compared to cats, dogs, non-buddhists. This is also explained in Tripitakaya, but I heard it from the sermons I've listened.
Hope this answers your question!