It's pretty simple.
Just download the Shimeji app
And the Puro Sprites
Click on the shimeji app and hit start. Go to import, follow the instructions and import the file that says "Shimeji". If you have questions don't be afraid to ask.
(He does like to be in the way of your keyboard sometimes)
You can't yet since it's still in development, BUT you can join the development studio and help out with the game here: https://scratch.mit.edu/studios/29190573/
( Fun fact: this is a picture from the actual game I'm working on in Scratch 3.0! If you wanna join the studio and add fun stuff to it, here's the link: https://scratch.mit.edu/studios/29190573/ )
For phone (no iPhone)
App: https://play.google.com/store/apps/details?id=com.xerophi.shimeji
Got from a YouTube video.
Here's a link for sprites that you import in this Shimeji app. Basically, you need to unpack a .zip file, go into the app, press the "+" button and import a folder that you got from a .zip file.