If you've seen my basic sounds tutorial (and if you haven't, you should!) you'd know a rimshot sound instead of a snare. The classic snare, while considerably more tricky, is far more versatile.
Step 1 What's it sound like?
The classic snare is similar to the word "poof" if said without the "oo". So, to start, say "poof", then take away the vowels. You should have a sound similar to a spit, almost as if you were trying to spit tea leaves from the tip of your tongue.
Step 2 Fine-tuning
Once you have the "pf" sound down tighten your lips up to get the nice, crisp snare sound. Alternatively you can use your tongue to get a similar sound, although that method will cause trouble when learning more advanced beatboxing techniques.
Now you can use it in place of a rimshot in your beats for a more realistic sound. It's also possible to hum whilst making this sound, but I'll talk about that in a later tutorial.
Described as "[one] of the finest exponents of the hip-hop discipline" by YouTube's music blog, Fat Tony's tutorial series is THE definitive guide to learning how to beatbox. As featured on Humanbeatbox.com and BeatboxAustralia.net.