View the video to see what you will be making.

Go to the Shape section in the Create panel and create a Rectangle in the top viewport with Length 150 and Width 80 . Right click and convert it to an Editable Spline. Select all vertices and click Break. Add a Wind spacewarp in the front viewport and set Strength: 20, Turbulence: 100, Frequency: 5.

Add the Garment Maker modifier to the rectangle and set the Density to 0.4. Add the Cloth modifier. In the Cloth modifier open the Object Properties. Select the rectangle in the list. Set it to a Cloth Object and change the Preset to cotton. Click ok to close the Object Properties window.

In the modifier stack expand Cloth and select Group. Select the vertices along one edge and click Make Group. Name the group fixed. With the group still selected click on Drag. Drag causes the selected group to stay fixed in the scene so they wont move.

Click on Cloth at the top of the modifier stack to go back to the top level of the cloth modifier. Click on Cloth Forces and add the wind spacewarp to the simulation.

Click on Simulate and wait for the simulation to finish. After its finished click on play and there you have it its a cape blowing in the wind.