Supplementing with formula?

K • Mommy to Ava and Kieran 💕
My supply has taken a major dip (normally pump anywhere between 4-9oz per session, past few times I've been lucky to get 2-3oz combined) I have a strong feeling I'm gonna have to supplement with formula sooner rather than later (was planning on supplementing when I go back to work when baby is 4 months old) does anyone have any tips for supplementing?