Right click on the Asset Source or Asset folder in question, and select New subfolder:
![](https://superbig.co/uploads/images/writing/quick-tip-how-to-create-a-asset-subfolder-in-craft/Screencast-2017-06-01-at-10.16.58-AM.gif)
One problem that tends to come up a lot is that people doesn't realise it's possible to create subfolders in Assets.
Right click on the Asset Source or Asset folder in question, and select New subfolder: