Once you collect enough Monster Cells from a specific Monster, you can use the Crafting Pods from the Lab to start the creation of the Egg.
If you don’t have enough Cells you can use Elementium or Elemental Cells to fill the gap!
But be aware that if you don't own the Monster that you want to craft, you must have at least 80 normal Cells to use the Elementium and the Elemental Cells.

To craft a Water Element Monster, you can use both Monster Cells and Water Cells.

If you already own the Monster and you want to craft a second one you can use any amount of Elementium or Elemental Cells.
IMPORTANT: You must choose ONLY Elementium or ONLY Elemental cells, it is not possible to mix those two together to fill the missing gap.

Ranking Up:
To rank up a Monster using Elemental Cells, you will ALSO need a minimum amount of Monster Cells. Then you can use Elemental Cells to fill in the missing gap. Note that this applies from Rank 1 to Rank 5.

To rank up an Earth Monster from Rank 2 to Rank 3, you will need to have a minimum amount of Monster Cells, then you can fill the missing gap with Earth Cells (using Fire cells or any other Elemental Cells will not work).