(Warning: Long message ahead.)
I trained and supervised lots of Sensor subordinates in the military, and it's not that tough once you get the hang of it. For starters, as everyone else suggested, make things as concrete and/or linear as possible. For example:
If you need your assistant to type up a certain routine report in a standardized format, then give him a couple of past reports from previous periods to use as examples. That gives him something concrete as a starting point.
Another example: If you need your assistant to follow a certain procedure, then type him up a checklist of specific steps to follow with room to fill in information and/or check off completed items.
These are good supervisory tools. If he doesn't follow format on the report or he skips steps on the checklist, it's going to be easy to spot the error and point it out to him. They are also good supervisory tools because they force you as the supervisor to clarify what you want done and how, and to foresee and provide the necessary tools to get the job done. Unless your assistant has prior experience and expertise at a task, you can't just give him an assignment and tell him to figure out how to do it himself. He is going to have different priorities and ideas from you on how to do the job, so inevitably you're not going to get what you want. As a supervisor, it's your job to figure out for yourself how you want the work done and train/instruct your assistant properly. You're responsible for the final product, so the burden's on you to dictate the format and procedures. Drawing up procedures and formats for specific tasks will force you to be specific and clear about what you want accomplished.
Other supervising tricks:
Let's say you tell your assistant to type up a report in a standardized format, and you've given him some previous reports of the same type to serve as concrete guides on how to do the report. First off, give him a deadline for completion of a report. Since he's a P, that's pretty much absolutely necessary. When he's done, review the report. Inevitably, there will be some mistakes because he'll prioritize formatting elements differently from you and take shortcuts on some aspects that you may think very important. So mark up the report with a red pen showing where the errors are, sit down with him and show him the changes you want made (using the previous reports to provide a concrete example of the correct way to do things), and then give him the report back and have him make the changes. Give him a new deadline for the changes, and review the report again when he's finished and red pen it again as necessary. Keep having him make changes until he gets the work right.
A couple key features in this process: 1) When you find errors, don't finalize the report yourself. Make sure he makes the changes himself. That's how he learns and remembers. 2) Don't hang over his shoulder and do the work or the changes with him. You want him working independently as much as possible. If it's a short task, the only time you want to see him is at the beginning of the task when you give him the assignment and at the end when you review his work. (And of course you want to be available if he has questions.) If it's a long project, you should establish some intermediate deadlines and review stages to see if he's staying on schedule and check that he's following procedures right. But aside from those instances, you want to stay away from him in order to avoid the impression that you're hanging over his shoulder or checking up on him constantly. The process of review and returning his work for changes is inevitably a tough one for his ego, and it will add insult to injury if you're demonstrating a lack of trust by constantly hovering over him and watching everything he does.
Also, make sure he's clear that you're the boss and have the right to set deadlines, enforce them, review his work, turn it back to him, etc. You don't have to be accusatory or tough on him when he makes mistakes. Just remind him that you are responsible for the final product, so you have to have the final say on how it's done. And it's his job to keep doing it until it's done right.
By the way, I wouldn't call him a co-worker. Use "assistant," "intern," "subordinate," "direct report," or whatever you want. It's not derogatory. It's just a reminder that you have the final say over what happens in the office. He may in fact have expertise in some areas and may be able to teach you some things. That's fine. You don't have to treat him like an idiot. But if you're supervising him properly, then that means you're giving him orders, setting deadlines, and turning back his work on a daily basis. So your titles should reflect your additional responsibilities. Don't send mixed messages by feigning an artificial equality that isn't really there in practice.
Over the longer term, figure on training him hard on one new procedure or routine report per week. And once you start training him on it, make sure he always does it from then on. Just set deadlines and review his work each time he does the task, until you're absolutely certain he has it down perfect. Ultimately you want to reach a stage where you can take a vacation and he can keep the office running at least with regard to all the routine tasks. Policy decisions remain your responsibility, of course. But if you supervise and train correctly, there's no reason he can't handle all the routine stuff himself.
Just to try to close up here:
It may sound like I'm treating Sensors like real idiots here, with all this talk of step-by-step training. But actually they only need supervision (and lots of it) at the earliest stage. Meanwhile, they're sharp and they notice and retain details. Eventually, once they've picked up enough procedures and responsibilities around the office and start to feel a sense of ownership of the tasks, things will click for them. They're noticing and inputting information of their own, on top of what you're teaching them, and one day you'll see that they're actually more expert on the office functions than you, and furthermore they've imbibed the philosophy of the office enough that they can even handle and design new projects on their own.
But sometimes it takes a fairly long period of good supervising and training to get them to that point. And you're the one who has to do the initial work. You can't just wind them up and set them to work. You have to concretize what you want done in the office (create specific tasks, define the procedures by which they are carried out, and specify exactly how the results should look at the end), and then you have to enforce deadlines and monitor the quality of your assistant's work. You have to be willing to be the boss and deliver bad news occasionally.
Also, realize that you may have to accept some compromises on quality of work. No matter how much you dog the assistant, you're probably not going to get him to do things exactly the way you would do them. So in order to complete the process of handing off a task to him and finishing up training on that task, you should be willing to accept some strategic compromises on quality. Don't insist on perfection. That's part of the process of delegation of authority and responsibilities--if you give a task to someone, to some degree you have to let them have some discretion over how they do the work. Besides, you can always do some additional quality control training at a later date. Right now, it sounds like you just need to get some tasks off your desk and onto his desk, even if it means you have to make some compromises on quality for that to happen.
That's just my own experience, of course. Hope that helps. Good luck!