What are bookmarklets and what are they good for?
Bookmarklets are "Bookmarks" (Chrome, Firefox) or "Favorites" (Internet Explorer/Edge).
In contrast to regular bookmarks they will not lead you directly to another page. Instead they will execute a script on the current website.
A bookmarklet can do everything in a page that a developer can accomplish with JavaScript. It will be executed within your session as if yourself were doing it. So when you're logged in to a system, the script is as well.
So bookmarklets can actually be dangerous.
Never use a bookmarklet that you did not create by yourself (or if you have the slightest doubt in the person recommending a bookmarklet to you).
A bookmark that instead of an URL (starting with http://...) contains a Script starting with "javascript:"
For example: javascript:alert(document.title);
A bookmarklet can utilize everything on the page by using JavaScript.
Advanced bookmarklets may download code and run little applications on your website.
But since bookmarklets are potentially dangerous I would refrain from using such advanced bookmarklets offered by someone unknown.
Enable your boomark bar. In Chrome it's Ctrl+Shift+B.
First: have a javascript snippet at hand. It must be executable on the browser console without errors and show the desired results.
Remove all comments from the code if you have any. Take care that every line is closed with a semicolon.
Second: open bookmarklet generator
Copy your JavaScript snippet into the text box, also give your Bookmarklet a name (this will show in your bookmarks)
A button to your right shows your Bookmarklet name. Press it and it will execute on the generator's page. Drag & Drop it to your Bookmark bar.