Work on getting some Pets down the 2 columns on the left side of the game.
Clear these columns and Pets will slide towards and down them.
Each of the other 7 columns can have one Pet in it at a time. Try to keep removing blocks in them so that the Pets will fall.
At the same time, use block matches or the built up rockets to get the key down.
Choose when you'd like to open the key.
In the video example I got about half the Pets first (the matches in the second column were going well!)
But the key can be opened at any time, some players found it easier to do it early in the moves.