Start from the main screen.
Tap SINGLE PLAYER.
Highlight the world you want to share. (Tap it.)
Tap SD CARD.
The game just backed up that world to your internal memory card.
Quit the game and start your file manager (I recommend X-plore).
Browse to the internal memory and find the folder SURVIVALCRAFT and open it.
You will see the world you just backed up.
Post that file online using whatever you have - Box or Dropbox...
(If you use dropbox, the game can sign in automatically.)
In that case, don't tap SD CARD, tap DROPBOX instead, but it won't make a backup on your memory card.
Then use the box or dropbox app to copy the url to share with us...
Hope that helps.