The good news is that you don't have to create them. I've already done the hard work for you! A round-up of all the relevant information I've provided online is available on my blog, The Proofreader's Parlour in an article entitled Roundup: PDF Proofreading Stamps (quick-access links). Here you'll find links not only to the stamps files but also installation instructions, some advice about making the best of the stamp tool in Acrobat (and other PDF editors), and a link to some musical notation stamps (rather niche, I know, but useful to some!).
One caveat is that these are compliant with the UK's British Standards Institution (BSI) 5261 mark-up language. For UK readers, these will be perfect. For those outside the UK, it's worth checking to see whether certain symbols differ from the industry-standard mark-up in your own country. The website page that I listed above also includes a referring link to a site for American editors and proofreaders looking for stamps files that are specific to the US. I've not used these so can't comment any further on their usability, but I'd recommend readers do a comparison with those and my own proofreading stamps.
There is some further supporting information in the Stamps Archive on my blog. This includes tips on ergonomic PDF proofreading (including making your own quickie stamps using the snip tool) and a link to a book by Thom Parker on using stamps in Acrobat. You'll have to scroll around a bit because there's quite a lot of information in there!
Hope that helps you (and saves you some time)!
Louise
www.louiseharnbyproofreader.com