How do you encrypt a file or a single folder worth of data?
There are plenty of utilities that will create a simple encrypted volume/image for you, I've used Exces, but I got it in a bundle, otherwise seems a little bit expensive. I know there are others. Exces does what it says and nothing more. 

It creates a volume where you can drop anything (folders, files, etc.). 

After you close it, you need the password to re-open it and access its contents. If you lose it, you're hosed.