It’s explained extensively on ih8mud.com

3/16 double flair

One of these makes life way easy!

https://www.amazon.com/TGR-Professional-Brake-Flaring-Degree/dp/B01HOXH242/ref=mp_s_a_1_8?crid=FWEJ3SIX31NI&keywords=brake+line+flaring+tool+kit&qid=1665968758&qu=eyJxc2MiOiI0Ljc4IiwicXNhIjoiNC4zOCIsInFzcCI6IjQuMDEifQ%3D%3D&sprefix=brake+line+%2Caps%2C109&sr=8-8&ufe=app_do%3Aamzn1.fos.f5122f16-c3e8-4386-bf32-63e904010ad0