Ethereal Island
Welcome to Ethereal Island!
Before anything, to start working on this island you must have at least 2 different single element Ethereals. This includes the following: 
Ghazt, 
Grumpyre, 
Reebro, 
Jeeode, and 
Humbug.
They may be teleported to the island at level 15, which costs exactly (per monster) 
1,310,640 treats. As for how to breed them, each is bred using the 4-element monster on each island paired with any 3-element monster. Example: on Plant Island, breeding an Entbrat and a T-Rox may result in a Ghazt! Note, Ethereals are rare and hard to breed. It might take awhile before you get any at all!
Populating the island 
Once you've started teleporting single-element Ethereals, you can start populating the island! This is a pretty self-explanatory process as by this point you should know how to breed. Ethereal Island is limited to only single and double elements, so attempting to breed in anything higher will result in... nothing! As previously stated, Ethereals are very hard to breed for, and there is no exception to Ethereal Island. The breeding time for the singles is shorter, but the doubles are very hard to come by the first time. It might take you a long time before finishing the song!
Farming the island 
Now we can start making some shards. Ethereal Island has farming methods that range from beginners to professionals, with the difference being the rarity of monster you're farming. Note, you're going to want to upgrade your castle for more monsters if you decide to farm! For early game, any unaccompanied beds and grid space should go to as many single Ethereals as possible. Here is an example of an Ethereal Island that holds 173 monsters. Higher levels and happiness = more shards!
- These grids are visible here
You may also use this grid, for full collection purposes. Two of the 3x3 spots arent necessary for full collection, as you only need 5. 3 for Dragongs, and 2 for tree/mine. These may be filled however you like.
- These grids are visible here
For a good start, fill the grid with as many common single-elementals as you can in every space labeled as 1. The best monsters for this are Humbug, Jeeode, and Reebro, as they make more than Ghazt or Grumpyre. Any 1x1 gaps fill with dipsters. (if you're early game, just do leave these blank.) At later stages in the game, these can be swapped out for 
rares and 
epics. 
Monculus (and its variants) is the best monster to farm due to its low bed count, and high currency production. However, it is quite rare to breed. You might be better off sticking with single ethereals and their variants, but if you manage to breed any Monculus variants they are very good.