It is actually very simple now. First start up tor, then start Bitcoin Core with the -listenonion flag and -torpassword=<pass> flag where <pass> is your control port password if you have one set. In order to use it though, you will need to enable the control port in the torrc file.
For clarification, by "now", he means Bitcoin Core 0.12. For earlier versions, you will need to configure Tor to run a hidden service. Instructions can be found
here. For a Bitcoin node instead of a webserver, change HiddenServicePort to "8333 127.0.0.1:8333". Then, find your onion address in the hostname file, go into bitcoin.conf and add externalip=youraddress.onion, along with listen=1 (assuming Bitcoin is already configured to connect through Tor). It can take a while for new hidden services to show up on the Tor network, so be patient.
Right, forgot to mention that. However, Bitcoin Core 0.12 will be released very soon. Probably within a day or two, so if you want to do it the easy way, just wait a little bit and wait for the official release. If you are impatient, you can download the binaries from my github at
https://github.com/achow101/bitcoin/releases/tag/v0.12.0. Those were deterministically built so you can check the checksums at
https://github.com/bitcoin/gitian.sigs under 0.12.0 and check that the downloaded file's hash matches the signed hashes of all of the people who have done gitian builds.