Making the Portal
To make a portal to the Twilight Forest, make a 2 x 2 shallow pool (4 squares) of water in a grassy area, and surround it entirely with natural stuff (flowers, mushrooms, saplings or tall grass). Then throw a diamond in (with the "Q" key, by default) and stand back!
If the portal is not working, the mod may not be loading properly. On the title screen of Minecraft, there should be a "Mods" button. Clicking that button will give you a list of the mods that are loaded. As long as the Twilight Forest mod shows up in that list, then it has loaded with no fatal errors.
If the mod is loading properly, double-check your portal. Are there natural things in all 12 locations (including corners) around the pool? Is the pool only one block deep? Those are the most common problems.
What to Beware of
Although a lot of the Twilight Forest is rather tranquil, some places are definitely unsafe. The portals do not have any particular safeguards. It is rare, but you could arrive in the courtyard of a rampaging Naga, in a Hollow Hill, or even on top of a tall tree. Be prepared to defend your portal, or to flee if necessary.
How to Get Back to the Overworld
You can make a portal back to the Overworld in the same way as you initially made the portal to get to the Twilight Forest. Diamonds appear in the Twilight Forest at the same rate, and at the same depths as in the Overworld.
Your respawn point in the Twilight Forest will be at the same coordinates as your respawn point in the Overworld. That doesn't mean that it will be near your portal, or anywhere recognizable. For that reason, it may be advisable to either make your portal near your respawn point, or to place and use a bed near your portal soon after you arrive.
Progression and Adventure
Some tech mods or ore mods have a tree or ladder structure, where you have to build items from one stage in order to reach the next stage. In the Twilight Forest, you can roughly measure your progress by combat difficulty. Since most of the forest is peaceful, the difficulty is centered around the various landmarks. This guide attempts to rank the landmarks by difficulty, and to break them into tiers, so the organized adventurer can be prepared for what awaits. The first thing you want to do is craft yourself a Magic Map.
The landmarks in the Wooden Tier are fairly easy, and can be attempted with little or no armor, and simple wood, stone, or iron weapons. Hedge Mazes are a good place to start out, as most monsters die in 1-3 hits from any weapon. The treasure chests inside frequently have food or other useful supplies for your journey ahead.
Small Hollow Hills are a good introduction to the Hollow Hill concept, and can help you build a supply of iron for armor, and coal for torches. They can be somewhat claustrophobic compared to the Medium or Large hills, but that can work in your favor, by breaking up packs of monsters, preventing them from swarming you. Small Hills are also a good place to find Glowstone, if you haven't crafted a Magic Map yet.
The Iron Tier landmarks are where things start to get rather difficult. It's recommended to have iron armor or higher, becuase the Naga is waiting. The Lich Towers are filled with hidden treasure, and the dangerous Lich awaits you. Medium Hollow Hills have a little bit more strong enemies, but more rewarding treasure, including redstone, lapis lazuli, and gold.
The Diamond Tier landmarks are extremely dangerous. It's recommended to have diamond, Naga Scale or Steeleaf armor, as the powerful Hydra awaits. The Labyrinths have extremely rewarding items and dangerous monsters, along with the terrifying Minoshroom. Large Hollow Hills have some of the greatest treasures, but also have the most threatening monsters. The Dark Towers are filled with hordes of frightening monsters, along with the scary Ur-ghast that waits for you to come to the very top, but also have very valuable items.
This is accessible upon defeat of the snow queen in her ice tower.
Twilight highlands Troll cave Obsidian vaults Giant cottage Thornlands Final plateau