If you use a normal browser: drag the normal html2canvas bookmarklet to your bookmarks toolbar.
If not use the following link: bookmarklet for those special mobile devices
click / tap that link and then bookmark the page, edit the bookmark and remove the start up until including #_remove_this_
part at the beginning of the URL, it must start with: javascript: to be correct.
If you are using Firefox and the NoScript Addon: disable the ABE part of it, took me quite some time to figure out that the reason for an unreliable bookmarklet was in NoScript...
If you are a developer and want to debug locally (you need the source tree of your html2canvas at:
http(s)://localhost/html2canvas/)
check the following box to get the bookmarklet patched automatically ;)