Hanko is almost always with one's last name. There is a kind of hanko that uses both first name and last name, but most people in Japan do not use it unless the occasion is as serious as purchasing a house. An average Japanese person either does not have this type of hanko or uses it once or twice in life time. We pick characters that can phonetically represent the names. There is another approach you might want to consider.
Some English name have meanings.
For example, consider a name like Blackwood . For a name like this, we can try a different approach, which is to pick characters that mean black and wood . With this approach, people in Japan cannot know how your name is pronounced, but they will know what it means instead. Usually, round hanko is used by individuals. Square hanko is used by organizations.