If you're new to fountain opening, it can seem like a daunting task. However, it is actually quite easy. This is a short guide on how to do it.
Step 1) Find some sort of sharp item.
Step 2) Focus your determination onto the item.
Step 3) Leap into the air and then stab the ground with your now determination-infused item!
Step 4) Get out of the room as quickly as possible.
That room will now have a dark fountain in it!
If you're a darkner, you cannot open dark fountains (in fact, you are almost definitely already in a Dark World if you can read this). Instead focus on protecting the fountain in your Dark World, because if it gets sealed, it's all over for you.
Before opening a fountain, give a piece of glass to an object in your room. This will empower it and make it a Secret Boss.
Open fountains in rooms that have easily reachable large doors in them. These will turn into Grand Doors in the Dark World, and allow you to escape the Dark World using them.
After you open a fountain, speak to the leading darkner (or darkners) in the Dark World and forge an alliance with them (although make sure that there is a Grand Door to escape from so you are not trapped forever).