Hi!
Welcome to X-Cart. This mod is described in the KB:
http://kb.x-cart.com/display/XDD/Step+3+-+applying+logic+changes
You can skip theory and go straight to the mod example here:
http://kb.x-cart.com/display/XDD/Step+3+-+applying+logic+changes#Step3-applyinglogicchanges-ApplyingchangestoX-Cartstore
Also, before you get started you would better to read about empty module creation here:
http://kb.x-cart.com/display/XDD/Step+1+-+creating+simplest+module
Sure, if there are any questions, please feel free to ask.