In addition to what Dave said, once your stakeholder determines what he wants in it (or if that's your choice even better) separate it into what do they need to KNOW and what do they need to leave this training being able to DO. Then how imperative are each of those things. That helps prioritize resources. Then I take a look at what prerequisite information/skills do they need to execute the objectives. Is it extensive enough to require it's own course? Self study? Instructor led? Webinar? Depends on the material. This let's me determine the scope of the whole training curriculum.
What I do not do is try to take a person from novice to expert in a single online course, which can often be what stakeholders are expecting.
The project I'm currently working on is a decent example. The "entry level" material is in elearning to build a foundation. The "Need to KNOW" material is referenced but included as Resource Attachments. There are required hands-on practice exercises to reinforce what was in the course. Then the more complex elements are included in On the Job Training. Managers/supervisors are given a manual of topics to cover with the employee and any needed support materials. Elements that require remembering clear steps have job aids for reference. Yes, they can be completely useless. But they can also be very effective when they are designed the way an end user would design their own "cheat sheet".
If yours is a topic that experienced employees already know, check with them to see what kind of materials they have created on their own. Even if you don't replicate them, they can give great insight into what areas need job aids. Beyond that, the less formal the better. When someone is referencing a job aid to get thru a task, it needs to be very user friendly or else they end up like you said, useless.
I got good results from a set once that included many pages. Our team edited the steps down to the bare bones, formatted them into post card size, had them laminated and bound on a ring. The most important part was the clear table of contents. The user could scan it, flip to the correct page and be on their way.
When I'm feeling overwhelmed, I back up and make sure I'm looking at it from a high enough level to start with. Like Dave said, map out those objectives. Then see how they need to be broken up into individual/support topics. I do not start looking at detail content until I have the program structure laid out. What will the end users need to be successful? Then what are the different ways I can give them that? What would be the most effective? What is feasible given the time and budget? If I can't go with exactly what I'd like to do, what is another way I can present it? Time and budget limitations can trigger some serious creativity.
I'll stop there. This is a great topic and while I hope you get the help you need, I'm also interested in seeing what else the community adds.