How -- click on the Recruiting tab and select FSS. Then click on the state(s) you want to buy. There are 10% and 15% discounts if you buy enough states. When you are ready, click to purchase.
When -- anytime you have enough money. If you have carryover cash you can do it immediately. Otherwise, you will get your recruiting cash as soon as the job application period ends and open signup begins. That happens the night before recruiting starts.