Boot a live and create
- /boot partition , size should be ca 100 MB
- /root partition and if wanted
- /home partition
Boot the installation medium. During
Click on the partition created for boot and tell it to mountpoint /boot, ext2 filesystem
Click on /root partition and set it "use as physical volume for encryption", no mountpoint
If wanted the same for a /home partition
At the start page of partitioning click on "configure encrypted volumes" and hit "finish"
Back at the start page of partitioning click on the encrypted volume[s] and add the mountpoint[s].
If wanted you can disable to overwrite old data (which, depending on the size, will safe _a lot_ of time).
Click done setting up partitions and go on like usual.