For plotting routes through trees, a handheld gps is rarely better than 20 feet either side of your intended route and points. GPS is normally very poor under tree cover.
I've had to position points in thick woodland a few times before selective availability came off GPS.
I used a sighting compass (military style prismatic at that time, I use a suunto now. it is less affected by rain and dust).
First job was to set the difference between grid north and Magnetic north by siting from a known point to a distant known point. Do not trust the figures given on the map, I've found some which were way off both magnetic and GPS alignments. 20 degrees error for one Kenyan map.
After that, mark the firing point and intended backstop position onto as large a scale map as you can get (google earth is a bit dodgy for that. if you can't get a good map, you may be able to get ortho corrected air photos) and measure the bearing between them from grid north.
Start by sighting from one of those points and pick the first tree that is on your line, walk to it, tie a coloured streamer around it and go to the other side of that tree and repeat the process. once you are through the wood, do the same thing the other way and keep repeating until things look right when you come out of the wood each time.
I've had various size excavators and a D9 fell trees. A big gum tree makes one hell of a slap when a D9 knocks it down towards you!