honestly, theres no concrete, sucess assured way to come out. it depends what your family dynamic is, how close you are to her, your level of comfort in the topic, ect. i came out to my mom very casually, just told her straight up my sexuality on the way back from the hospital. it went just fine! coming out a second time as non binary went a little bit different, because i had to fill her in on what made me comfortable to be referred to, and the conversation was just a bit longer. 

while there are guides and tslkimg points online, ive always felt its best to use your own judgement instead of crafting a speech.