The pattern for the garment tells you how large to make
the thing prior to felting, and the heat and agitation in a
washing machine does the rest.
The agitation you need is best obtained in a top-loading
washing machine because the front-loaders are too gentle.
The one in the photo at the right is an old wringer washer
we bought at an auction for just this purpose.
If you try this at home, make sure to put the garment in
a pillowcase or lingere bag to keep the fibers that are
released during the felting from clogging up your machine.
Add a little detergent, which helps the felting process.
Check the results every so often (it will probably take at
least 15 minutes), to see how much shrinking has occurred.
If one wash cycle is not enough, skip the rinse and spin
cycles, and run it through another wash cycle.
When the garment has reached the desired size, squeeze
out the excess water and let it dry. If you are making a hat,
forcing it onto a metal mixing bowl of the right size helps
maintain the shape you want.
