There are many different ways of retting. The retting process is rotting of the inside reedy layer of the flax, so that you can free out the fibers.
The Germans use the process of dew-retting, in the fall of the year, when the dew is the heaviest. It is laid out in long thin rows in short grass to collect the nightly dews. It may take several weeks. The problem with this method is that you get mold and mildew on the flax, so the quality is not as good as the Irish linen.
One of the other ways people may be more familiar with is placing it in tubs, but we haven't found 18th century references to this, and it may be a modern (and more environmentally friendly) method. Here on the 1730s-40s Ulster Irish farm, we use our creek.
This was repeated until it was all thrown in the water.
How will you know it's ready? You pull out a few strands, dry it, and see if it breaks cleanly. When you wrap it around your finger, there should be snaps at every bend. This frees out the fiber, and it should come away in long strands.
When you first start retting your flax in early summer, it's rather pleasant to spend time by the creek. But as the summer wears on, the smell of the rotting flax and the muck in the creek bottom becomes very foul. The smell is like a broken sewer pipe. In fact, the stench is so bad, that, according to folk lore, if you had anything to hide, you'd bury it there because no one would come close. Also, according to folk lore, only young single men were allowed to work at the retting ponds, and this is an age when personal hygiene wasn't well-known, so you can only imagine what a horrendous stench it was.