Be sure there is ice in the water circulator (if used).
Fill a round bottomed flask no greater than half-full.
Connect to the bump trap with a plastic clip.
Lower the flask into the water bath to submerge the liquid (don't submerge the plastic clip).
Turn on the vacuum source (vacuum will hiss).
Rotate the flask at a moderate rate (one-third the maximum value).
Close the stopcock in the apparatus (hissing will stop).
Evaporate until solid forms or liquid level doesn't appear to change anymore, then evaporate an extra few minutes for good measure.
To stop evaporation, reverse all steps: