Tone Ag Consulting Ltd.

1. Save Current track (N/A if you’ve just started up the GPS)
a. Press the Menu button twice
b. Choose tracks
c. Choose save (if Save is grayed out, go to step 2)
d. Save entire log
e. Choose OK for the default name
f. Choose Clear, Yes
2. Set the track to record to the accuracy you want
I’d suggest 0.01 km for most single-field measurements, but this will fill up the track log fairly quickly.  Note that Saved Tracks cannot have more than 750 points.  At 10m per point this would mean a maximum distance of 7.5k or 4 miles per track.
3. Walk/drive around the area to be measured
NOTE: If the track crosses itself, results may not be accurate.  Go around the perimeter of the area.
4. Save Current track
a. Press the Menu button twice
b. Choose tracks
c. Choose save
d. Save entire log
e. Choose OK for the default name
f. Choose Clear, Yes
5. Read acres
a. Press the Menu button twice
b. Choose tracks
c. Choose the track you just made, press Enter
d. Area is displayed under Area.  The units can be changed by going up to the currently displayed units and pressing Enter.

(initially published on my personal blog)