This howto log is done on Ubuntu 7.10 64-bit version but should be fine with most debian based distros

Go to the related link at the end of this article if you want to know what the commands exactly do or if you dont have the same system as I have

Create a partition GParted (aptitude install gparted), Select Filesystem as "not formatted" and note down the device name (e.g. /dev/sdb1)

Install needed tools

 aptitude install cryptsetup

Load needed modules

If you have a 32-bit Pentium, you need to load aes-i586 or simply aes (not optimized) instead of aes-x86_64

 sudo modprobe aes-x86_64
 sudo modprobe dm-crypt
 sudo modprobe dm-mod

Create LUKS Filesystem

Kernel 2.6.24 (Hardy):

 sudo cryptsetup -c aes-xts-plain -y -s 512 luksFormat /dev/sd<Partition>

Kernel 2.6.20 (Feisty):

 sudo cryptsetup -c aes-lrw-benbi -y -s 384 luksFormat /dev/sd<Partition>

Kernel 2.6.10 (Dapper und Edgy):

 sudo cryptsetup -c aes-cbc-essiv:sha256 -y -s 256 luksFormat /dev/sd<Partition>

In My case the output after Feisty command was

 root@y:~# cryptsetup -c aes-lrw-benbi -y -s 384 luksFormat /dev/sdb1
 WARNING!
 ========
 This will overwrite data on /dev/sdb1 irrevocably.
 
 Are you sure? (Type uppercase yes): YES
 Enter LUKS passphrase: 
 Verify passphrase: 
 Command successful.

Formatting your crypto partition

Open it first (decrypt)

 root@y:~# cryptsetup luksOpen /dev/sdb1 somename
 Enter LUKS passphrase: 
 key slot 0 unlocked.
 Command successful.

Format it with an Ext3 Filesystem

 root@y:~# mkfs.ext3 /dev/mapper/somename
 mke2fs 1.40.2 (12-Jul-2007)
 Dateisystem-Label=
 OS-Typ: Linux
 Blockgröße=4096 (log=2)
 Fragmentgröße=4096 (log=2)
 122109952 Inodes, 244189623 Blöcke
 12209481 Blöcke (5.00%) reserviert für den Superuser
 erster Datenblock=0
 Maximum filesystem blocks=4294967296
 7453 Blockgruppen
 32768 Blöcke pro Gruppe, 32768 Fragmente pro Gruppe
 16384 Inodes pro Gruppe
 Superblock-Sicherungskopien gespeichert in den Blöcken: 
         32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
         4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968, 
         102400000, 214990848
 
 Schreibe Inode-Tabellen: erledigt                            
 Erstelle Journal (32768 Blöcke): erledigt
 Schreibe Superblöcke und Dateisystem-Accountinginformationen: erledigt
 
 Das Dateisystem wird automatisch alle 26 Mounts bzw. alle 180 Tage überprüft,
 je nachdem, was zuerst eintritt. Veränderbar mit tune2fs -c oder -t .

and close it

 root@y:~# cryptsetup luksClose somename

Create open and close scripts

Create a directory where you want to mount it to and dont forget to give you write permission to that.

openCrypto.sh

 #!/bin/bash
 sudo cryptsetup luksOpen /dev/sdb1 somename
 sudo mount /dev/mapper/somename /mnt/somename

closeCrypto.sh

 #!/bin/bash
 sudo umount /mnt/somename
 sudo cryptsetup luksClose somename