From 1922:

Put a pinch of hops and dessertspoonful of sugar in a basin, just before dinner, and drain the boiling potato water on to them. Put aside one potato to mash with a little of the old yeast, and after the meal the “hop-tea” will be cool enough to strain and add. Cold (or lukewarm) water may be added if the bottle is not quite full. Leave 48 hours if possible – 24 in hot weather. This will suffice for eight or nine pint measures of flour, leaving enough over to start the next bottle. It is not necessary to tie the cork, which need only be laid in to keep out the dust; but it is a good plan to have it tethered to the neck of the bottle, so that when the rising scum pushes it out it need not be retrieved from some remote or dusty corner.