If you've imported a shape to Inkscape, it may already be a path. Press F2 or select the node editor tool. When you click on your shape, it should show the nodes which define the shape. If you've imported an image, ignore the above and see the following:
Your need to convert the image to a path implies that you have a bitmap, rather than a vector. Select the image, use Path, Trace Bitmap. This will create a second object atop the bitmap. Drag it out of the way and delete your original image.
Note also that the trace bitmap feature will trace the inside and outside of your shape. You will have two paths and the "inside" will be the thickness of your line. If you can recreate the image to be completely filled, you won't have to deal with the inside of the trace.
If you can't recreate the imported image, deselect the fill for the new object and ensure that your inside trace is continuous, allowing you to use Path, Break Apart and delete it.
You should then be able to accomplish that which is presented in the video you linked.