A festival of Osiris involves dressing in black and sacrificing an ambrosia of flavored beer and oil at an altar surrounded by growing crops.
A character could be required to grow any five out of:: cabbage, leeks, garlic, onions, carrots, barley, and flax. This means that you must use an altar that has both grass and sand next to it (garlic, leeks, and onions grow on sand; cabbage, carrots, barley, and flax grow on grass). Many altars will be suitable, but most areas are likely to have a designated altar where everyone meets and can support one another.
The flavor of beer used in your ambrosia is the only variant for this festival. Osiris beer requirements consist of a combination of two properties and a flavor -- for example, "Potent Black Barley" or "Fruity Sweet Honey". Having extra properties in the beer is okay. For example, "Potent Fruity Sweet Black Beer, hint of barley, hint of date" would satisfy "Potent Black Barley". Note that "Very Potent" and "Potent" are two distinct properties, and thus "Very Potent" does not satisfy a "Potent" requirement.
List of all possible Osiris beer requirements
Suppliers of Osiris beers can be found by checking the Osiris Inventory page - make sure you specify to your supplier that you want an Osiris ambrosia.
Arrive ahead of time and grow your flax and barley. Weed the flax and then leave it be. It will stick around for a long time in that state. Grow the barley to completion in the normal way with applications of water, fertilizer, and weedkiller. Once barley is "Ready To Harvest" it will last twenty minutes, plenty of time for several people to perform the ceremony.
See also: The Test of Festivals, Determining Festival Requirements, Festival Schedule, Festival Altars, Festival Needs, Osiris Inventory, Hathor Inventory, Beer Inventory.