I have a client for whom I've designed a newsletter. I did this in Photoshop, since it's the program I'm the must accustomized to (I know InDesign would've been the "right" choice). And now I need to hand over a file that can be used by my client as a template for writing these newsletters. How do I do this in a way so that anyone with just basic computer knowledge can make them? In Microsoft Word/OpenOffice for example. The file is a bit image-heavy so it needs to be versatile.
If they require a template, then the best option is probably to build the template in the application itself, using other apps like Photoshop to create assets like logos you'll reuse.
What's the newsletter for? If it's to be emailed, then you're probably better off creating a MailChimp or Campaign Monitor template. Emailing Word documents often isn't a good solution.
To expand on Marc Edwards response, if you're going to email this out to its intended recipients you should probably create an HTML template. MailChimp allows you to upload html pages with images as a .zip file.
If it's a print template you should probably be considering using something else - Photoshop is a fine piece of software but it's usually the wrong tool for tasks such as document layout (I've seen some fantastic posters done in Photoshop but they were almost more of a full-page image). If you're going to be doing this kind of thing you may want to become more familiar with some of the other pieces of software in the CS. It may be that you end up using Illustrator (great for vector art or single-page layouts) or InDesign (definitely a page layout solution) and sending a .pdf off to your client.