It's been more than a decade since I did this, but I can give you the basics. The key is to start and end the clip at a point where the sound pattern temporarily hits zero (the horizontal axis). I used Audacity to do the editing. It had a feature that would find those crossings automatically, which made the process somewhat easier.
Make sure you don't encode as MP3, as it doesn't support seamless playback in a standard way. There's a forced gap due to the compression. Use for example wav or ogg.
I've seen Mixcraft used to make loops before on different streama, but have no personal experience with it. It seems pretty easy to cut and splice audio sources though. Similar to doing it in Vegas for video. It has a fully functional 2 week trial though if you want to give it a try. I've just always used Audacity for audio editing.
I'd probably go the same way, I'm not one to spend money when I don't have to, even if it means a few extra steps. The only reason I even got Vegas was because I was doing a ton of heavy video editing monthly for a show.
It depends a bit on your operating system. But I suggest audacity as a cross-platform, easy to use, but powerful audio editor. As long as your song has a common section to loop from you should find this very easy to do.