Designing suit by cutting cloth from a model I am new to blender and I'm trying to make my traditional clothing but I am facing a trouble here.

I cut the cloth from makehuman model that I created and I am trying to make this "triangle open" on the chest which I can't. I tried deleting faces and filling them with the "fill face" function but it didn't go well.

Here is my attempt:
You can try the Knife tool: in edit mode, press K, then clic ond the start point, end point, enter to confirm.
Start and end need to be on a pre-existing vertex or edge.