Here's how I did it, and it's really quite easy:
1. Download the RPMs for fpc-1.9.5 and fpcsrc-1.9.5. Use Alien to convert these to DEBs, then install them in the usual Debian way. You may find that this fails to create /etc/fpc.cfg, in which case you have to get a copy of the template for this file and hand modify it to suit your system. You will certainly need to add the path to the Gnome libraries in the 'library' section - e.g. -Fl/usr/lib
2. Download the latest Lazarus daily snapshot source and unpack it to an appropriate directory, or (better) get it via CVS. I have it in my home directory, as this eliminates problems of access permissions.
3. Make sure that you have the necessary development packages installed on your system. gtk-dev and gdk-pixbuf-dev are probably not installed by default (I'm not sure offhand if these are exactly the right names).
4. CD into your lazarus directory and type 'make' without the quotes. If this fails with a linking error, then you probably need to install some more development package(s).
If the build succeeds, then you can type ./lazarus to run Lazarus. If it fails and you can't resolve it, come back and ask.
Eric