Changing time signature

How do you change the time signature partway through a piece? For example, most of he piece I'm writing is in 4/4, but there's a waltz section later on. For now, I'm writing it in 4/4 and ending up with really weird measures, but it's really difficult to play and to relate the notes to how it should sound... it's doable, but annoying nonetheless. Can I change the time signature for one section only? If so, how?
Thank you in advance :)