“I tried standing on it and immediately fell flat on my face.”
You tried to use a new device that some people use to ride around on. The device requires good balance. You tell a friend that you tried to use it, but you got hurt.
I tried standing on it and immediately fell flat on my face.
"Immediately" can mean right after meeting someone, right after seeing something, etc.:
The very first time I met her, I knew immediately that she was the woman I wanted to marry.
When I saw him in that costume, I immediately started laughing.
To "fall flat on your face" means to fall down straight, without breaking your fall with your hands. "Falling flat on your face" sounds painful and dramatic. English speakers sometimes use this phrase when telling a story, to make the story sound more exciting and dramatic.
He was walking out of the door, and he missed one of the steps on the porch, and then – BAM – he fell flat on his face on the sidewalk.