Licenses themselves are often licensed under separate terms which don't have much to do with their own. The GPLv3, for example is licensed under the following terms:
Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/
Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
Which is definitely not a free-as-in-freedom license. You need to check for each license separately which conditions apply to the license text itself. In the worst case the license text is not licensed at all, which implies All Rights Reserved which means you are not allowed to use it for your own project, because that would be an illegal reproduction of the copyrighted license text.
But even when the license of the license allows modification: Unless you are a lawyer specialized in copyright law you should not modify an existing license. A license is a legal document, and writing legal documents should be left to the professionals. Otherwise you risk writing something which doesn't actually say what you want it to say.
Another reason to not invent your own license is to prevent license proliferation. There are already lots and lots of open source licenses with very similar terms, but yet they are often incompatible just because they say so. Inventing a new license even though an existing one would do makes it harder for all of us to keep track of all the licenses available and is a hinderance to developers who would like to combine code licensed under different licenses in one project. So unless you are sure your new license fills an important niche not yet covered, please try to use one which already exists.
Regarding "approval for new licenses": A license is a legal agreement between you and your users. Nobody needs to approve it except for you and your users for it to be legally valid (except when it contradicts local laws - that's another reason why you should let a lawyer write them). However, when you want others to use your license for their FOSS project, you might seek approval from a quasi-official organization like the Free Software Foundation or Open Source Initiative. Their recommendation might not have any legal weight, but it confirms that your terms are in agreement with their definitions of Free Software / Open Source Software.