I had a smooth installation but it was a bit a hassle to identify the correct procedure (because I wanted more space for /boot than suggested). A few more hinzs of how to create a custom partition scheme would be helpful
To make a shortcut for you, the trick is to click on the three dots in the header, on the second screen (Installation method).
There you can choose to launch the storage editor.
Please do not forget that changes are made immediately. Pls. backup your data if you use a existing installation.
We are discussion on that here to: