That's simple! <3 There are two ways:
WAY ONE: Once you're in lonley cave, you'll be inside a den, make your way outside of it and you'll see a portal coming almost out of the water. This portal leads to Cape Of Distant world, so make your way there. Then, when you arrive to Cape Of Distant World, turn 180 degrees clockwise (behind you) and you'll see another portal which leads to Flourite Plains. :3 If you are still interested going to another path, here it is.
WAY TWO: When you're in lonley cave, turn to your right, you'll see a black materialized portal leading to ficho tunnel so make your way there. Once you arrive there, follow the water. You'll cross a water fall and a large lake with a Fluorite Crystal in the middle. then when you keep going, you'll see a dead end with another black materialized portal, this leads to fluorite plains.
