I wanted to encrypt my Documents. My home folder is on a logical volume. So I made some space by removing another logical volume and creating a new one, which gets encrypted by cryptsetup. Here are the commands:
lvdisplayDisplay all logical volumes on the system
lvremove defaultvolumegroup/dataRemove the logical volume "data" from the volume group "defaultvolumegroup"
lvcreate -L 53.66G --name data defaultvolumegroupCreate the logical volume "data" with a size of 53.66 GB in the volume group "defaultvolumegroup"
cryptsetup create cryptodata /dev/mapper/defaultvolumegroup-dataCreate the encrypted device "/dev/mapper/cryptodata"
mkfs.ext3 /dev/mapper/cryptodataFormat the device with the ext3 file system
mount /dev/mapper/cryptodata /home/jeltsch/dataMount the device