There are plenty of docs on the net to get one started with lvm and encryption on Slackware, this is more of a quick bullet list to get you there in 0 time.

Boot up from the cd and start the following once at the prompt:

1. fdisk /dev/sda ( create one boot partition 100mb and another partition to fill the drive )
2. mkfs.ext4 /dev/sda1
3. cryptsetup -y -s 512 -c aes-xts-plain luksFormat /dev/sda2
4. cryptsetup luksOpen /dev/sda2 slackcrypt
5. pvcreate /dev/mapper/slackcrypt
6. vgcreate slackvg /dev/mapper/slackware
7. lvcreate -n swap -L +5G slackvg (if you are using an ssd , put swap on something else)
8. lvcreate -n root -l %FREE slackvg
9. mkfs.ext4 /dev/mapper/slackvg-root && mkswap /dev/mapper/slackvg-swap
10. run setup and install to your lvm partition and also use the boot partition as well
11. after installation chroot to your mounted root partition run mkinitrd, use /usr/share/mkinitrd/ or run your own
12. update grub or lilo ( I use grub2 )