I'm kinda shocked no one mentioned slide masters and PPT templates (pot / potx).
These two PowerPoint features should help make the PowerPoint sufficiently flexible if someone were to edit it later. Infact the whole idea behind these were to build templates.
While I'm not very sure of any standard way of doing this, here's how I would go about building a template
1. Enlist the different kinds of slides your client would typically use:
There might be some intent behind why the client would use the deck. Like make sales pitches, submit reviews, internal communication. You would be able to make a list based on this.This would basically involve listing out what kinds of slides he would typically use. For E.g. The would be a cover slide, divider slides, Index, Table of Content, Agenda etc. Also I'd also keep a "blank slide" incase there's something the client would wanna do, outside of our list.
2. Decide designs for each kind of slide
If you're planning on directly incorporating you designs in PowerPoint make sure you refer to the third point first, else if you're more comfortable with another graphics software, would do the conceptualizing there first and then bring it to PowerPoint
a. Decide on fonts,color etc
You would want to think about aspects like typical color scheme, fonts etc for the entire template before starting any designing.
b. Think about typical elements that would go into different slides
These would include charts,tables,titles,paragraphs,shapes etc. It helps designing these commonly used elements first
c. Build different layouts
Based on the list be made earlier (of different kinds of slides), we would conceptualize designs for the cover slide, table of content slide etc. We would also be leveraging the elements we designing in the earlier step here.
3. Incorporate these into a template
Here, instead of making a PPT as usual, go to view>slide master first.
This would change the interface a bit, you would have a "master slide" along with layouts below it. Every slide you designed in the previous section would be a new layout here. When the client is using your template, he simple needs to rightclick on a slide and hover over layout to get a list of different templates to choose from.
Thus you can create different layouts for different slides. The overall slide master interface should be pretty self explanatory. Another major benefit of slide master is, it allows you to handle all fonts, colors, font sizes etc in one single place. Thus forcing the client to maintain consistency.
4. Save it as a template
Once you're done with all the designing, close the slide master view and save it as a potx.
I might add more points to the slide master section in a while, but most if it is a lot like conventional PowerPoint, and should not be that tricky.
5. Train your Client
Last but not the least, no matter what you do, it would be crucial to explain and train the client like a baby.
I've had times when I made a decent template that was then butchered by the user's "creativity". They will mostly feel tempted to play around and get creative sometimes, this would likely result in your design breaking.
A simple session on design do's, dont's (and why do and dont) would save you a lot of frustration IMHO.