Minecraft wiki entry for Raid_farming, associated text: A village (1 bed and 1 villager are sufficient).
A surface on which the raid is forced to spawn (the ground, or an isolated surface above). Raids cannot spawn on leaves, scaffolding, or liquids; this can be used to help localize the raid spawn point.
A way to funnel or guide the raid mobs into the killing area.
A killing area; that is, mob grinder to kill the mobs, or a holding area for the player to kill them and earn experience as well as loot.
A way to get Bad Omen again, so that the next raid can start:
A pillager outpost converted to a pillager trap to allow the player to kill raid captains that spawn in the outpost. This is actually a type of pillager farm. In Bedrock Edition, this type of farm yields mob drops but no raid loot.
A way to force the raid captains in the raid out of the raid context, so that they give Bad Omen when killed.