This document outlines the necessary steps to get you up and running with SagePay iframe forms.
Go and get the Custom checkout templates from here.
Extract the files into one of your local directories.
If you're on a windows machine just run Kernow.bat (a good old double click will do).
If you're on a mac clone this gist and copy the file Kernow.sh into the Kernow folder and then run it on the command line ./Kernow.sh
You should see a simple GUI running.
Inside the unzipped folder you'll find some more documentation. Just open CustomTemplatesReadme.html in your browser.
Mostly we'll be wanting to embed card detail forms into checkout pages via Iframe.
To achieve this we need to use Low profile templates, mainly card_details_low.xslt and authorisation_low.xslt.
Follow the instructions on the documentation as these use the main xml files.