How do I tell my mom that I'm Bisexual?

Just tell her you need to have a talk. When she comes to talk, sit ner down, look her in the eyes and calmly tell her, "Mom, I am going to tell you, right now, the truth. I am bisexual. I like both girls and boys."
And then don't walk away, don't look down, have complete confidence. If you waver, she won't think you're sincere or serious about this, and just "playing with the thought," and will therefore shoot you down. Talk about it, and tell her the complete and honest truth, See More but in a simple way that doesn't take a long time.
There, I tried to help, however, I may not be tbe best at helping in this situation, since I'm straight. I just have a lot of gay and bi friends. I think 95% of my friends group is either gay, bi, or this one person, who's a trans. I hope this helps.

flat out say it straight.......

Do what I did.
Go home and drop the truth down.