I took parts of an MIT licensed repository and modified them, added some files of my own and now want to release the entire project under the GPLv3. How do I ensure the multi-licensing is done properly? Currently I’ve put a copy of the original license as “license-original.md” and the GPL as “LICENSE” in the root directory of my project. Is that enough to release the code or do I need to do something else as well?

I went through some links: What to do when using MIT-licensed code in a GPL-licensed project?

https://opensource.stackexchange.com/questions/1980/is-it-possible-to-release-a-project-under-the-gpl-if-it-contains-mit-licensed-li

However they do not cover how to actually do the “keep this copyright notice” bit.