In order to enter the Twilight Forest, one needs to create the Twilight Forest Portal. The simplest portal consists of a 2x2 area of water source blocks, surrounded by Dirt, Coarse Dirt, Grass, Podzol, or Mycelium, with any flowers or mushrooms on the surrounding blocks. See the images for examples of how Twilight Forest Portals are made. Once the portal is set up, throw a Diamond into the water, and, if set up correctly, lightning will strike the portal, and the water sources will be replaced with the portal blocks.
Any blocks can be underneath the portal, but there must be blocks below the water source blocks. Additionally, the Diamond may be placed in the water before the flowers are fully around the water sources. The portal can be scaled up to having an 8x8 water source, and can be any shape smaller than 8x8. To deactivate a portal, either break one of the portal blocks, or destroy one of the blocks around the edge or below the portal. Destroying the flowers have no effect on the portal after it is made. The portal does not have to be open to the sky, and can be created whether day or night. The biome the portal is in also does not effect the portal.
All items that can be used around the edge of the portal are: Poppies, Dandelions, Azure Bluets, Alliums, Blue Orchids, Oxeye Daisies, Red Tulips, Orange Tulips, White Tulips, Pink Tulips, Rose Bushes, Lilacs, Peonies, Brown Mushrooms, Red Mushrooms, Grass, Double Tallgrass, Ferns, Large Ferns, Mayapples, any type of Sapling, any type of Leaves, Dead Bushes, Vines, Lily Pads, Giant Lily Pads, Giant Water Lilies, and Thorn Roses.
When creating a portal in the Overworld, the exit in the Twilight Forest will be as close to the same coordinates as possible while only in the Twilight Forest, Dense Twilight Forest, Firefly Forest, Twilight Clearing, Oak Savanna, or Spooky Forest biomes. Portals created in the Twilight Forest in other biomes will link to the closest portal in the Overworld that already exists. Using this can be useful for very fast travel, by linking portals such that they would be in gated biomes in a way that has multiple portals closer together that warp the player to somewhere near the edge of the biomes, enabling the player to move from one edge of a gated biome to the other without covering too many blocks by foot.