You’ll want to start with a clean glass, since any soap residue will remain in
the glass once you freeze it and can lead to an unpleasant taste mixing with
your beer. Make sure your mug is at room temperature, since introducing hot
glass to a cold environment could lead to cracked or broken glass.
Clear a place in the freezer for your beer mug, either on the flat shelf or on
the door shelf, and make sure it has good air circulation and isn’t touching
anything else.
You can use a dry glass, but know that it will take longer to freeze than a wet
glass. Plus, it will likely result in a plain frozen mug, not a frosty mug. The
frosted look will be most visible when there are water droplets readily
available for freezing.
Wet the outside of the glass with cold water, and set it in the freezer.
Condensation will form on the mug, and combined with the cooling environment,
it’ll turn the water droplets into ice crystals. If you open and shut the
freezer door several times, you’ll introduce brief inputs of warmer air; this
will help build up the ice crystal layers and give your beer mug a nice frosted
appearance.
The length of time it takes your beer mug to develop a frosty look will depend on
many factors, such as freezer temperature, room temperature, how full your
freezer is and how much frost you want on the glass. However, you can expect to
have a frosted mug in roughly 20 to 60 minutes.