Use the right mirror pronoun to match the subject:
with the slingshot.
by selling their house.
Reflexive pronouns may be used to emphasis.
He himself finished all that work. (it's significant)
I myself couldn't believe what you said. (Again there is emphasis)
"By" + reflexive pronoun = alone, without help
My sister lives by herself. (on her own / alone).
I prepared the dinner by myself (alone).
I solved the puzzle all by myself. (emphasized)
Some other uses
Did you enjoy yourself at the party? (Did you have fun?)
Please help yourself to some food. ( Go and get whatever you want)
How do you feel
See Other Pronouns