The question "Which software program do you use to write stitch guides?" comes up all the time. I thought I'd link to Linda Reinmuller's blog which lists eighteen graphing programs at the bottom of the page. The links will take you to the program's website which often has demos you can try out. Note that many of the programs will only run on Windows machines.
Linda doesn't cover all the programs available, just the ones created specifically for stitch diagramming. I use my Mac word processing software called Pages to create my stitch diagrams, but it has limitations such as not being able to create curves or dimensional shapes easily. I also find it very hard to number stitch diagrams with it and haven't figured out how to layer a copyright notice on top of the grid yet. But mostly it works for me.
I know a great many people (John Waddell used to teach a class on doing this) who use Excel to create diagrams. I haven't tried it myself but I'm pretty sure it will work fine.
Other Mac users (and a lot of stitch guide writers use Macs) use Canvas X which is no longer supported by ACDSee, so modern Macs can't run it unless they are dual Mac/Windows machines which can run the later versions that are Windows only. A new edition called Canvas 14 (I think) is supposed to have come out last spring. Adobe's Illustrator is also quite popular but these two programs are very powerful graphics programs, with big price tags and a steep learning curve, so only the most determined users go this route. I believe Illustrator is a subscription only service now which a lot of people don't like.
I hope this is helpful for all stitch guide writers Out There.