Use the chainlength guide at parktools, they have several methods.
Use a chain tool. Since you only need to remove links, you can just punch them out using a hammer and nail on top of a vice, but a chaintool is a useful thing to have.
Practice on some old chain links first.

Count twice. Look at your master link and see how it works, they are all tool-free. After removing the master link, remove the excess links from one end but take care to leave the correct link type for rejoining the masterlink. Generally, the masterlink connects 2 innie links so don't leave an outy link at one end.