There is a technique to opening up the rear derailleur so that the wheel and cassette go in easily. It's actually easier to do than it is to describe but you definitely do not want to derail the chain off the crankset as chain tension helps. Basically:
- Shift into your smallest cog before you remove the rear wheel, that will make it easier to reinstall the wheel and to get the chain onto the cassette.
- When it comes time to reinstall the wheel stand facing the back of the bike with the bike saddle near your left hip.
- Using your left hand you reach down and swing the rear derailleur cage backwards, this motion may be difficult at first but it helps if you also press down on the small tab that's halfway down the derailleur cage between the two pulleys. Some downward pressure on that tab while you use your hand to swing the derailleur body (the parallelogram part) backwards away from the frame will 'open up' the derailleur and create a large open loop in the tensioned chain (that's still on the chainrings up front).
- While holding the derailleur body in the opened up position with your left hand bring the wheel in with your right hand (bike supported by left hand and saddle resting against your left hip) and slide the cassette up and into that big open chain loop and set the chain onto the smallest cog (which is why you shift into that smallest cog before removing the wheel)
- Work the axle into the dropouts, secure the quick release tightly, shift back into an appropriate gear while turning the pedals with your hands and lifting the rear wheel off the ground and you're good to go.
That motion with the left hand to open up the derailleur cage and pressing down on the small tab midway between the pulleys (which can be done with the thumb of your left hand or by reaching across with your right hand) is the key. If you can't figure out this motion then stand in the same position facing the back of the bike with the saddle against your left hip before removing the wheel and try rocking the main derailleur body (the parallelogram part) backwards and forwards with your left hand to see the articulated motion. It's easier when the wheel is installed as the cassette holds the chain loop open but seems more confusing when the chain is slack (which is where pressing down on the small tab helps).
Sorry but I don't know of a video that shows this technique but there probably are some out on the net if you look around. Again it's much easier to do or to watch than to describe.
-Dave
P.S. The method I use is a lot like what this guy demonstrates: https://www.youtube.com/watch?v=n1qFwFDzp3U