If you end up eventually having a lot of sheets, and images, it might be a good idea to have a load state. The process of putting together a loader can be a little involved, but there are many methods and events that help with the process I will not cover that here, but I did write a post on that.
In any case using a canvas solution is pretty easy to get started with a method that might look something like this.
At which point I can then use the key ‘sheet-block’ when making sprites. For more on this topic you will want to check out my post on making sprite sheets from canvas.
In some cases I might not even need to bother with a sprite sheet at all, and just use a singe generated image. To pull this off then making a sprite I pass an instance of phaser bitmap data to the sprite in place of a key to a sprite sheet. I cover this in further detail on making sprites with canvas.