The hike is beautiful and very rewarding. It is muddy, but you will enjoy a lot of fun. Don´t pay money to the lodge, it is a scam. There is a beautiful, well-maintained path with bridges and stairs which is completely FREE. Just go to the parking lot by the waterfall, followed the path to the "La Fortuna" sign and you will see a few stairs to the jungle. It is an official path to climb the volcano. First, you need to cross a small river (bridge) and after a few minutes, you get to the meadow (don't be scared of cows, they are friendly). Then there is an intersection, I recommend going straight to the jungle (not to the right, even if there is an arrow). Soon you will find stairs and a bridge, so you can be sure that you are on the right path. After some time both paths will become one anyway, but the straight one is nicer.
At the parking lot by the waterfall, to the right of the Fortuna lettering, there are a few stairs. If you follow the path you come across a cow meadow to the hiking area!