Select manual partitioning and create two new partitions in the free space:
- A small one (about 500 MB), which you format with a Linux file system (e.g. ext4). Select
/boot as mount point for it.
- For the remaining space, choose "Use as: physical volume for encryption".
Afterwards, go through the process of configuring encrypted volumes (in the top of the manual partitioning screen). A new volume will appear above the physical one. If you just need one partition besides
/boot, format it and mount it as
If you want to create more than one encrypted partition (this includes swap space, which should be encrypted as well on an encrypted system) or just retain some flexibility, create an LVM on the encrypted volume. Therefore, select "Use as: physical volume for LVM". Just like with the encryption, you will have to "Configure the Logical Volume Manager". Then you can create logical volumes, which will again give you additional volumes above the encypted and physical ones. Format and mount those as desired.