Maori 1

For Casual and Ladder games, you can't. The system combines your sets with the sets of your opponents and picks from those.

For Friends and Practice, you can by using the Set Builder, where you can make a set of Things you want to play with. The FAQ has more details:

mythicalgame.boards.net/thread/3/set-builder-work

To be specific to your issue:

1) Create a new set.
2) Give every Thing you want to play with 3 stars.
3) Give the Things you don't want to play with the "X".
4) Save the set.
5) In Play Practice or Play Friends choose your set from the Set Picker button in the upper left.
6) Fill in up to 3 open sets with AIs or Friends.
7) Start the game.

By the way, in Practice and Friends, you can reshuffle the set show using the button with the 3x3 grid next to the Set Picker button.

Hope this helps!