Taking a line starts at your side and under control.
Find a straight edge to keep your dog from veering in another direction.
Your dog must follow your hand motion in the line that will take him straight to the bumper.
This is a controlled drill. Start with short distances so the dog can see the bumper.
When your dog understands the concepts in the above videos and demonstrates he can perform them under control, you can add bumpers.
The advanced lining video below shows what you can eventually accomplish by properly teaching a dog to take a line. Your ultimate goal is for your dog to take a proper line to a 30 yard bird, an 80 yard bird and a 200 yard bird. So beyond the dog understanding that he has to go in the direction that your hand points, he has to get comfortable in holding that straight line at a long distance away from you. Do not try a lining drill at these distances until your dog has mastered the beginning stages or you will just confuse the heck out of him and frustrate yourself.
Basic hand signals are the next step in teaching to handle.
Step 2: Basic Hand Signals Extend the Art of Handling