Easiest way to get baby to supplement w/ formula?

Bianca • I'm 19, married & raising two girls👨‍👩‍👧‍👧 Genesis🦄 DD: July 24, 2014 Actual birthday: July 27, 2014 & Leilani🦋 DD: June 22, 2017 Actual birthday: June 10, 2017

Don't know if that made any sense but my daughter refuses to latch on, I can only pump & give it to her in the bottle. Pumping takes a lot out of me, especially in the night! I was wondering what is the easiest way to supplement her with formula! What brand? How much? Half & half? But I tried similac I think that's how it's spelled but it's rough on her stomach.. I know I should ask my ped but I want other peoples opinion!