How to remove supervisor password on older Thinkpads

If you lock your 2008-2010-era Thinkpad with a supervisor password and forget it, there is not much you can do. Unplugging the battery won’t help (although I have heard that supervisor password on some models can be removed this way) and getting Lenovo to fix it for you can be time-consuming, especially compared to how easy to service and how well documented these machines are.