You want practical? Get on the TOS platform, put on a calendar and then adjust the greeks one at a time -- time, volatility, market price and see what happens. Remember that a change in market price usually affects volatility, so that is tricky. Next step is to follow an actual (simulated) trade over a week or two, and take note of the change in the greeks and what happens to the position. I got a calendar on where there is almost no movement in market price, but the position lost money. That means the vol difference between the short and the long changed. Maybe there is earnings coming up, or some big event and the near month vol jumps up while the back month stays the same.