In either case because even though it is not part of the project does not mean you can ignore it. On my current project we are expected to document, but as usual we are given no time to accomplish that goal. Here are some ways to block that time and get your documentation done.

Add the time you will need to do documentation into the schedule

This sounds easy but in reality it is the hardest thing to do. From working in the computer field for 20+ years, I can tell you that telling your customer that you will need X hours in addition to the time to install to do documentation, will be a hard sell. Remember your customer could not only be the client, but could be your manager, or your PM. The key to this, is to sell them on the benefit that documenting this process will bring to them. This could be many things, faster installs in the future, better problem resolution, not being reliant on a single person (you) for problem resolutions, lower cost of training. The reason will have to be tailored to the audience, but to get time to do the documentation you will need to show some benefit for what in the past they were getting for free.

Add the hours to your estimate of the time spent on the project.

This may sound a lot like the previous point, well it is. The main difference is in how it is presented to the client. There are going to be clients that want a breakdown of every minute you are working on a project, and there are some clients that just want to bottom line, How long will it take?. In that instance you add your documentation time into your estimate for completion. If you know it is going to take 8 hours to complete the project, and you guess it will take 2 hours to document, then 10 hours will complete the project.

Is there a documentation Genie?

Um.. no. After you get buy in from everyone, then you just have to slog through the process of documentation. One of the best things you can do is to take lots of notes during the install process, NEVER rely on your memory as a note taking apparatus. You will forget things, usually minor things, but they will make your documentation incomplete. Set aside some time that you will not be disturbed. Writing documentation means you are reliving your install process and you will need a quiet time to process your thoughts and get them in the right order.

Writing documentation is never fun, but keeping a well documented process will help in the long run.