To please Maat, you must surround an altar with 21 flaming ritual torches. At this altar, sacrifice an ambrosia made from honey and the petals of a player-specific Rose of Ra.
Petals of any Ra flower may be harvested, which kills the flower completely. You can also test the flower to see if it works for your Maat requirements. As such, this is an indirect way to see what the color of your flowers are. The part of the test that varies per person is the type and color of the petals needed. One example might be "Any Giants" and another might be "leaves: magenta".
Suppliers of Maat Festival items may be found here. Additionally, there is a database of which flowers meet which requirements.
Note that the festival requires Ritual Torches, not glass torches. Ritual torches cost 1 canvas and 1 board with the Ritual Item Construction skill to construct and require 1 oil to soak the torch, after which it can be lit. Ritual torches burn for a few minutes (10 minutes) and then disappear. Maat altar riggings are thus temporary, though some designated altars may be pre-prepared - watch for announcements of usable altars.
It is possible to salvage the torches before they finish burning for a chance of recovering canvas and boards, but do not do this before all in attendance have pleased Maat.