You can do it on the B2 directly, but it is sort of, well, not elegant. You can also do it on your computer, using freeware such as Audacity, and then Import the albums onto the B2 via USB, or upload them via the WebUI, copy them over directly to the B2's HDD via NAS.
Either way, you'll need a phono-preamp if your turntable doesn't already have one built in. There are many new ones that have a built-in preamp, and some even have a USB connection to be used on your computer. (But that USB connection won't work on the B2. There you have to use the Aux input.)
Probably the number one reason not to do it on the B2 is that you cannot break the files into separate tracks. You can do that on your computer with Audacity (or such).
If you really want to do it on the B2, instructions are on the Brennan website,
here:
Aux In and Recording