If you look at the second video on this tutorial page under the section "learning to TIG weld" you will see how to do it, in the video we were welding on flat plate so the little back step at the end was enough on its own without filler, if you are working in a joint you may just need a touch of filler to prevent a crater, note the way the arc is set to fade out gradually (downslope) and the gas is set to continue (post flow) also the torch is held over the pool to maintain gas cover until it has cooled.
Keep the torch moving until the weldpool has finished freezing. Say for example making small circles during slope out and crater fill/finish current or in the case of the final tie in a pipe/tube butt just keep moving