As a scribe you have two responsibilities--the scoresheets and the timer. Sometimes the judge handles the timer, but generally the scribe does it.
Scoresheets vary somewhat in appearance, but in general this is what they look like:
No._____ Handler________________ Dog__________ Judge___________________
The handler will usually identify himself and his dog to the judge just before he walks to the post. Unless the scoresheets are already filled out with the names of handlers and dogs, it's best, if you get a minute here and there, to fill in that info from the running order for the future runs as much as you can, so that you're not scrambling to do it while you're adding up the last competitor's score. That way you can just check what the handler says against what you've written on the scoresheet. Don't bother filling in the judge's name unless you've got time on your hands.
Often, the printed scoresheet doesn't exactly fit what's happening in the trial. For example, there often is no "shed" before the pen, but only a pen and then a single or split. (And if you're clerking for the novice classes there will be no shed or single and the number of drive points will be lower in Nov-Nov and usually Pro-Nov.) In that case, it's a good idea to run a line through the "Shed" row at the outset, so you don't confuse the pen points with the shed points. And if you're in any doubt about the number of points for any phase of work, ask the judge, e.g., "Are you scoring the drive at 30 points or 20 points in this class?"
Some judges will give you a running commentary of points off as they go along within a phase of work. For example: "That's two off, another two, one off--how many is that now?" With that kind of judge, you write down each little 2 or 1 as the judge says it in the space between the phase of work and the Points Off column, and then add it all up at the end of that phase of work and put the total in the Points Off column. (That space is also used for any comments the judge may want you to record--e.g., offline, circled sheep--so that he can mentally reconstruct the run if any question arises. Some judges do this; some judges don't.) Other judges will keep the running total in their heads, and just give you a number at the end of the phase of work. Typically the judge does not give you a final outrun score until after the lift; then he'll say, e.g., "Three off the outrun, none off the lift." Write those numbers in the Points Off column. Usually you'll have time while the fetch or drive is going on to do the subtractions to get the scores for the earlier phases; after you've done it for awhile it becomes automatic. But it's okay if you can't get to it til after the run; that's very common with newbie scribes. The judge will continue on in this fashion, giving you the points off for the fetch as the sheep turn the post, the points off for the drive as the sheep approach the pen, and then the points off for the pen and shed as these are accomplished.
Bear in mind that the score is always zero for the Pen unless a pen is successfully accomplished before time runs out; ditto for the shed or single or split. You already know this, but it has important implications for you as scribe. If time runs out before the pen is accomplished, for example, IMMEDIATELY write "10" in the Points Off column for the pen, and for any subsequent phases of work. It's a good idea to jot down "Time" somewhere on the sheet, or put a "0" in the "Time:_____" space if there is one, as well. If you get behind in your adding and subtracting, that will save you having to remember a couple of entries later whether the pen was accomplished or not, which is especially hard to reconstruct if the judge has given you a running total of pen points off which you have jotted down.
At the end of the run, finish subtracting points off from possible points for each phase of work, writing the remainder in the "Score" column. For example (duh!), if there were 3 points off the Pen, you'd write "7" in the Score column in the Pen row. Then add up the Score column and fill in the Total. It's a good idea to add up the Points Off column too, even though there's usually no blank provided for a total, because that serves as a check on your arithmetic -- the total of the Points Off column plus the total of the Score column should equal the total possible points (usually 100 or 110 in Open). It's a good idea to initial the scoresheet somewhere, if there's no space for the clerk's name (as there usually isn't) so if a question should arise later they'll at least know who was scribing at the time.
Usually they give you a little hand-held timer. Punch in the time the judge is allowing. Hit the start button when the dog leaves the handler's feet. The timer will beep when the time is up, and if the run is still in progress the judge will call time. Immediately reset the time for the next run. If the run is completed before time runs out, ideally you would stop the timer and record the time remaining in the "Time:______" blank, if any, but that is a frill; rarely will anyone care whether you do it or not. Immediately reset the timer for the next run.
In the unlikely event that the judge decides at some point after the Outrun is complete to give the competitor a re-run (e.g., a loose dog runs on the field and interferes), IMMEDIATELY stop the timer at that point and note the time. That's because the rules require that a re-run begin from the point at which the problem occurred, so the handler and dog will run as usual the second time, but the timing and scoring will begin at the point where they were interrupted in the first run; all scores before that point will be from the first run, all scores after that point will be from the re-run. So you restart the timer with the remaining time at the point where the earlier run was interrupted. This is very unlikely to happen, and if it does, ask the judge how he wants to handle it, but DO stop the timer (or at least note the time) IMMEDIATELY when the judge indicates he's going to give a re-run and calls the handler off.
Some judges are very chatty, and some are silent and preoccupied. Adapt yourself to the judge. Don't be the one to start a conversation unless he's made it clear he wants to talk. The one exception is if you're unclear about what he's said by way of a score; then you must ask for clarification immediately. On one memorable occasion, when a timid clerk was scribing for a judge who mumbles, she couldn't hear him very well and decided he said "Nine off the shed" when he had actually said "None off the shed." The handler, who had executed a perfect shed, was naturally amazed when she saw her score, but by the time she raised the question the judge had forgotten her shed, and a great kerfluffle ensued. If the clerk had just said "Did you say nine off?" or "How many off?" at the time, it could all have been avoided. Of course, you are likely to be able to tell a 10-point shed from a 1-point shed, but if you're in any doubt at all about what the judge is saying, ASK. If you're lucky, the judge will be a chatty sort, in which case you can now and then during a lull ask him questions about things that puzzle you. Keep in mind, though, that in my experience overseas judges often are a little fragile in their self-confidence; they're judging people they don't know, in a place where they don't know the customs, and they wonder if everyone is thinking they're doing a bad job. If you ask why something that looked good to you got a bad score, or vice versa, be careful to make it clear that you're seeking knowledge from an expert rather than criticizing or reflecting others' criticisms.
If you keep your eyes and ears open and your mind engaged, scribing can be a way of learning a lot about trialing, dogs and sheep, as well as providing shelter from the weather and the second-best seat at the trial. Enjoy it!