After primary fermentation is over and you have finished pressing the must, clean the fermentor and piston assembly thoroughly. Reassemble the fermentor valve and sanitize the fermentor and piston assembly. Then transfer the pressed wine back into the fermentor. Use some sanitizer in a spray bottle and wet the sidewalls inside the fermentor. Then basically you just push the piston down onto the surface of the wine and place a stopper and airlock in the opening.