Roll-Your-Own Primitives: Learn how in "The Do-It-Yourself Guide to Squeak Primitives" by Stephen Travis Pope at http://www.create.ucsb.edu/squeak/DIYSqPrims.html (Tim Rowledge in a comm. to the list 10-FEB-2003: sections 1,2,3,4 & 9 are still relevant but all the others could do with rewriting to match the vm plugin story.)
Named Primitives: Learn how to write plugins for the Squeak VM in Smalltalk using Squeak 2.3's nifty new "pluggable primitive" feature with Andrew C. Greenberg's precis: Named Primitives.
Specifications for Named Primitives: Version 2.6 of Squeak includes a new-and-improved named primitive compiler that makes it much, much easier to build pluggable primitives. Andrew C. Greenberg has posted preliminary documentation of the Specifications For Named Primitives. (Tim Rowledge 10-FEB-2003. This needs updating & completing. Ought to refer to using VMMaker to build the plugins)
Extending Squeak by Writing Plugins, by Ned Konz http://bike-nomad.com/squeak/writingPlugins.html - If you have questions after reading the description and need clarification ask Ned Konz; he is probably willing to answer questions.