I struggled for a couple of hours on just the basics of creating a package and getting it found.  No matter what I tried, I kept getting the annoying and uninformative error:
    Error in startup script: can't find package FooBar 1.0


Here is the bare minimum for a pure-Tcl package called FooBar:

   * Create a package source file called FooBar.tcl
   * Populate that file with the package code, following the conventions in [William Duquette]'s tutorial on 'Namespaces and Packages' [http://www.wjduquette.com/tcl/namespaces.html].
   * Make sure that the FooBar.tcl includes this command:

 package provide foobar 1.0

 
   * Create a pkgIndex.tcl file

You can use the pkg_mkIndex utility, but it is often easier to just write this by hand.  Basically, you need to call '''[[package''' ifneeded] to tell the package system about how to load your package.  The load script is going to be a '''[[source]''' command, using the variable '''$dir'''

 package ifneeded foobar "source [file join $dir FooBar.tcl]"

   * Put both FooBar.tcl and pkgIndex.tcl into a directory (probably named ''foobar'').  If you put the foobar directory into your Tcl library, then it will just work.
   * If you can't put foobar into the Tcl library path, then you can adjust the library path to point to its location.  If your foobar direction is in ''/some/where'', then set TCLLIBPATH to ''/some/where''.  You can also extend the search path in your scripts.

 lappend auto_path /some/where