1. find the spot on a map (I like Acme Mapper
, you can use Google Maps or Topozone or a paper map).
Figure out the exact latitude and longitude (on Acme Mapper, just click to center the map on your trailhead/whatever and read the coordinates from the box at lower right).
You want "decimal degree" notation - that's degrees and tenths/hundredths/etc of a degree, not not degrees-minutes-seconds-arcseconds and not UTM. Many sites like Topozone will display in whatever format you choose, or you can convert by hand.
For example, if you follow the link above you'll see that I marked the spot that I think is the Lye Brook TH: 43.16331 deg North, 73.03874 deg West.
(I'm not sure about this location, the topo map and the street map don't quite agree. On the street map, Glen Rd continues a little further south
, and that matches your directions better. The satellite view (recent, by definition) agrees with the street map, and this wouldn't be the first time a scanned USGS topo map was badly out of date, so I'm now convinced my first guess was wrong - the trailhead is a little farther south.)
2. Edit your page. Right near the top (below Name and so forth) you'll see fields labelled "lattitude" and "longitude". Enter the values you just figured out.
NOTE that West longitudes (or South lattitudes, should the need arise) need to be entered as negative numbers: -73.03874 (otherwise you're saying Vermont is near Kyrgyzstan)