Easy as it is. Just add the repository into your rep. list, update the package information and install Skype.

Open your list:
sudo vim /etc/apt/sources.list

Add this line at the end:
deb http://download.skype.com/linux/repos/debian/ stable non-free

sudo aptitude update

Install skype, answer yes to all question(but think for your self!)
sudo aptitude install skype

If you have some problems with audio(like I had) try this lines:

killall pulseaudio
sudo aptitude remove pulseaudio
sudo aptitude install esound
sudo rm /etc/X11/Xsession.d/70pulseaudio

(I'm using Ubuntu 8.10, but also works with Ubuntu 9.10)