Using a map in a forested environment is similar to using a map in any terrain. In fact, it becomes easier if the map is used in conjunction with a compass. In a forested area there are many obstacles and the only difficulty is to maintain a straight bearing. Choosing a landmark to follow until you get an opportunity to check your bearing with the compass can be difficult as the chosen object shall almost always be a tree and a slight distraction will require you to pull out your compass and restart the entire process of determining your bearing again.
However, if you take careful steps in the desired direction you should be able to follow your chosen path to the destination. What you need to do is to first draw a straight line on your forest map connecting your current location to the destination with an arrow pointing away from the present location. With the map of the forest in your hand set the compass along its longer edge congruent with the straight line between that of your current location to the desired destination.
Turn the compass sides with that so that the North South points coincide with that of the map. Using this setting lift the compass up and point in the straight ahead direction of your current path. If you find that the compass magnetic head deflects more than it did on the map then you shall have to change your sight of view to realign it. Once this is done chose a far-away marker as your target. This shall set the path for the next minute or two until you reach the marker.