Timing Belt and tension-er pulley issue

I am trying to set the timing on my friends car, but I am having issues with the tension-er pulley. I do not understand how to keep the two holes steady facing down while tightening the bolt. As tightening the bolt just turns the innards or the pulley. I have read that there is a tool for this pulley but have not been able to find any information on it. And I am sure that there is a way to fab up something without having to pay for an expensive specialty part I will only use a couple of times. Thank you for your help on advance.