You should give some details about the style of cape, if it is supposed to be animated, etc... There are several approaches, like modeling it, sculpting it, or using cloth simulation.
You can sculpt it:
- Prepare a vertical plane, subdivide it a bit:
- Use brushes like the Draw brush to dig or bump the shape (press Ctrl to dig, you can also try the Crease brush for more sharp details):
- You can also use the Grab brush to deform the shape. There should be even more convenient brushes in next Blender that will easily allow to bend the mesh:
Or you can begin with a Cloth simulation:
- Create a plane, give it a group at its top edge. Give it a Cloth physics, with for example the silk settings, in the Physics panel > Shape choose the group as the Pin Group. Select the body (here a simple cylinder) and give it a Physics > Collision:
- Play the animation. Stop it when you're glad with the result, and in the modifiers, apply the Cloth. Now you can edit it the way you want.