How can I wire a Spring wound timer to my existing thermostat ?

Wow, this is so vague I can't even begin to answer.

Please provide more info so we can try to help.

We will take a leap of faith and guess you want to shut off heat after so many hours. You could install the timer on the 120 volt side of the step down transformer or on the low voltage t'stat lead. Wouldn't it be easier to get a new t'stat? If this isn't what you are doing, please clarify.

Stop guessing what a poster is referring to, wait until more details are provided.