Update: TeX Live 2010 (and newer) includes all the prerequisites. So instead of manually installing all the necessary packages, you can just install that. For XeTeX, the fonts (currently) need to reside in the system font directory for easy access, so you might have to copy the XITS otf files to ~/.fonts
(or something similar).
Since it might help other people too, I'll post how I got the XTIS fonts to work. This was my first time installing TeX packages by hand, so if I did anything in a non-standard or incorrect way please correct me. (I made this post community wiki, so you can edit it.)
My system is Ubuntu 10.04 with TeXLive 2009 installed from the Ubuntu repositories. However something similar should also work on other operating systems (again, this is CW, so please add any instructions). I will install everything for use with XeTeX. There might be some changes for LuaTeX.
In the following, replace ~/texmf by the output of
$ kpsewhich --var-value TEXMFHOME
or
$ kpsewhich --var-value TEXMFLOCAL
First we will install current versions of some prerequisite packages.
$ mkdir ~/texmf
$ cd ~/texmf
$ wget http://mirror.ctan.org/install/macros/latex/contrib/fontspec.tds.zip
$ unzip fontspec.tds.zip
$ mkdir tex/xelatex
$ mv tex/latex/fontspec/ tex/xelatex/
The last two lines are necessary, because otherwise xelatex
will select the old version already installed on the system, see Andrew's solution to his question.
$ cd ~/tmp
(or whichever temporary directory you like)
$ wget http://mirror.ctan.org/macros/latex/contrib/expl3.zip
$ unzip expl3.zip
$ cd expl3
$ tex l3.ins
$ mkdir ~/texmf/tex/latex/expl3
$ cp *.sty ~/texmf/tex/latex/expl3
$ cd ..
$ wget http://mirror.ctan.org/macros/latex/contrib/xpackages.zip
$ unzip xpackages.zip
$ cd xpackages/
$ tex xbase.ins
$ tex l3keys2e.ins
$ mkdir ~/texmf/tex/latex/xpackages
$ cp *.sty ~/texmf/tex/latex/xpackages/
Next we will install unicode-math
from it's git repository. Alternatively you can download it from CTAN.
$ cd ..
$ git clone "http://github.com/wspr/unicode-math.git"
$ cd unicode-math/
$ tex unicode-math.dtx
$ mkdir ~/texmf/tex/latex/unicode-math
$ cp unicode-math.sty unicode-math-table.tex ~/texmf/tex/latex/unicode-math/
And now finally the XITS fonts. Instead of the git repository you can download them from http://github.com/khaledhosny/xits-math/downloads.
$ cd ..
$ git clone git://github.com/khaledhosny/xits-math.git
$ cd xits-math/
Now install the .otf
files. I did this with Nautilus (the Gnome file manager), but I think you can just copy them to the ~/.fonts
directory.
It only remains to tell TeX about the new packages:
$ sudo texhash
To test if everything went right, compile something like
\documentclass{article}
\usepackage{unicode-math}
\setmainfont{XITS}
\setmathfont{XITS Math}
\begin{document}
$\mathcal A = α²$
\end{document}
with xelatex
.