Start off by installing ant then download the iText buildfile.

If you're using sun j2ee then set the environment variable J2EE_HOME
to point to your j2ee installation. If you're using Tomcat then change
the line

<available property="j2ee.jar" value="${env.J2EE_HOME}/lib/j2ee.jar" file="${env.J2EE_HOME}/lib/j2ee.jar"/>

to something more like

<available property="j2ee.jar" value="C:\Tomcat
5.0.27\common\lib\servlet-api.jar" file="C:\Tomcat
5.0.27\common\lib\servlet-api.jar"/>

then do

ant download.site

ant jarWithXML

copy dist\iText.jar and dist\iTextXML.jar and add them to your CLASSPATH