Easiest way to do this using built-in OS X applications is to open Disk Utility, click New Image, name it, choose destination, select a maximum size, turn encryption on, and format it as sparse image (this way it only takes up as much actual disk space as the formatting and files you have stored in it total). You'll be prompted to select a password. Don't select "add to keychain" so you'll be prompted for your password each time you want to open it. It will then mount the image and you can drag files into it which will encrypt them. Eject the disk image when you're not working with the files within. Double click the image file and enter your password to access.
It's like a mini encrypted removable drive.
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