How I did it: I received a lot of comments about Esperanto on my language-learning web site Fluent Every Year, so I looked into this language one afternoon, and then I commented on Twitter that it looked so easy I could probably learn it fluently in one week. That became a challenge, and so I did. You can read about my one-week Esperanto challenge at my web site.
Lessons & tips: The most important tip I can give is to find someone else who speaks Esperanto. It's fun to learn an easy language, and to find yourself capable of speaking and understanding quickly, but it's not fun if there's no one to talk to.
Resources: I used Lernu.net and the Kurso software, as well as some other dictionaries, translators, and word lists which I mention at my web site.