make oneself at home

verb1

1. •
   To make oneself comfortable as if one were in one's own home.

   • If you were lucky as a child, you were fêted in households other than your own, encouraged to relax, kick back and make yourself at home. — 2007

phrase1

1. •
   An invitation to a guest in the speaker's home to treat the house as if it were theirs.