I'm not specifically familiar with your Garmin, but I'm doing this with
an old Garmin II (a cheap, basic model 3-4 years old) and an iPAQ
running GPS_LOG WinCE, but there were some cabling issues to address.
I needed:

a) Garmin GPS to serial cable (Amazon)
b) Serial male-female gender adapter or "sex changer" (Radio
Shack)
c) Null modem (Radio Shack)
d) Serial to iPAQ cable (eBay)

All together, these cost about $75. The elements are connected in this
order from the GPS to the iPAQ, which is stowed behind my seat. (I
don't look at the iPAQ in flight; for me it is a simple logging device
which takes the GPS data and converts it into an .icg file.)

The program and more information are available at
http://www.soaringpilotsoftware.com/GPS_LOGpage.htm