I made this cake myself.
I made this cake
We organized the party all ourselves.
We organized the party all
Come in, children, and find for yourselves a seat.
Come in, children, and find yourselves a seat.
You are writing in Active Voice, therefore, you do not use "by" when it comes to the agent.
The last sentences is okay, as it means that they find a seat for each child.
Student or Learner