How can i pair an iPhone 5s with an Apple Watch Series 3?

So, i had my iPhone 6s perfectly fine, when fell down the stairs, and now i am stuck with my old iPhone 5s, and i got an Apple Watch Series 3 from my Dad a few weeks ago, and i wanna know, how i can pair an iPhone 5s running iOS 12.5.6 with an Apple Watch Series 3 running watchOS 8.7.1

Thanks in advance!