Use Bias binding. It's easier than it looks. You actually pull the quilt almost straight to sew through the inside angle.
I was really nervous when I had a quilt with 90 degree inside corners, but it turned out well following these two videos.
https://www.youtube.com/watch?v=Be9dzI_X41w
https://www.youtube.com/watch?v=4VGNcDoI-R8&t=3s
Watson