To install all the ITI code, create a directory on which you want the ITI-related files to live. Then, move the iti-3.10.tar.gz file there, cd to it, and do:
gunzip iti-3.10.tar.gz
tar xvfp iti-3.10.tar
rm iti-3.10.tar
cd src
Then, edit config.h, specifically the two path names DATA_PATH_NAME and FONTS_PATH_NAME (see comments there).
You're ready to make the system. Type make. Finally, add
the ../bin directory to your path (see your own .cshrc).