Every child is different, which means there are a countless number of ways or ideas that you could achieve this goal, depending upon the child's individual needs.
Some things that have helped my past ADHD students & my own ADHD son:
Sensory diversion - try altering the amount or type of light in the learning environment. Sometimes natural light is more calming than florescent lighting. Adding a comfort grip to the pencil to change the feeling of it. Attach a swatch of cloth to the desk or work area for the child to run his fingers over. Altering the type of seat the child sits on so that some attention needs to be paid to remaining balanced.
Noise level & Audio diversion - is there a lot of noise surrounding the child during the time he/she is expected to concentrate? Talking, music, traffic... Classical music has been found to be soothing for some while distracting for others. Try changing the sound and noise of the surrounding area either by removing audio distractions or adding soothing sounds like calming instrumental music or white noise.
Visual diversion - what does the area look like where the child is expected to focus? Are there many bright colors, shapes, and designs? Are there people to watch, pets, or other moving scenery? Limit the visual stimulation to that which is calm, quiet, and relaxing. This will help the child focus on the task at hand.
Energy output - children with a lot of energy need to be able to expend that energy! If they are being forced to remain still for long periods of time they will likely fail at this expectation. Provide the child with either time to move and wiggle before the assignment or during. Is there a way to allow the child to move or wiggle while doing the task?
Fuel input - children with a lot of energy burn a lot of fuel. It is important to keep their tummies full in order for them to be able to concentrate and focus. Protein rich snacks are slower burning, allowing the child the most benefit from them. Try giving your child a snack just before they are asked to sit down and concentrate on something.
These are just a few suggestions, there are hundreds more. I hope this helps.