Answer: You don't. CafePress does it for you automatically. Froogle does not accept feeds from individual CafePress shops. They only accept them from CafePress directly and CafePress already feeds every shop and product to Froogle on a rorating basis.
There are limits to how many shops can be submitted each month, so CafePress constantly feeds shops and products to Froogle. It may take a little time before your products show up because of those limits, but they will eventually show up.
So, you don't have to do anything. And in fact there is nothing you can do. Just sit back and wait.
Froogle also finds products on it's own using it's own search engine. So be sure you have titles and descriptions for all of your products, since those will be needed for matching words when people search Google, Froogle, or any other search engine.