Typically, the Slate is handled by the Second Assistant Camera (2nd AC) or "clapper loader"
(more common in Europe) who fills in the Slate and hold it in front of the camera and claps the clapper.
Here's the way it's called on many shoots:
A.D. (Assistant Director): Roll Camera.
CAMERA OPERATOR: Speed.*
A.D. Roll Sound.
SOUND RECORDIST: Speed.*
A.D. Mark it.**
2nd ASST. CAMERA: Scene 25 Foxtrot, Take 2. (CLAP)
*Means the device is recording at speed.
**Means "Do the slate thing."
Once the the Slate is cleared of the scene, the Director can call Action at her discretion.
When slating any
shot where sound is being recorded, the person handling the Slate reads off the scene and take number before clapping. This ensures that the scene and take information is recorded on the soundtrack in addition to the clap for synchronization.
It's REALLY IMPORTANT to make sure the Camera and Sound are rolling for a few seconds before you "mark it" with the slate, otherwise you risk the slate not being on camera or the clap sound not being recorded.
Because the slate is designed to be easy to erase, it's also easy to erase things that don't change. As such, it's pretty common to stick tape to the slate with information that doesn't change much or at all.
THE TAIL SLATE
Finally there's one additional slating convention. The TAIL SLATE is done when, for whatever reason, it's not possible or convenient to film the Slate at the head of the take. In such a case the shot is done without a slate at the start but, before the Director yells "Cut", a "Tail Slate" is called for. The 2nd Camera Assistant does the Slate thing BUT to indicate it's a Tail Slate, the Slate is held UPSIDE-DOWN. (CLICK HERE to see an example of a Tail Slate from "Spock's Brain".)
Oh, and the thing used to wipe the Slate is called the MOUSE.