You can do it with the head on the car. You get an air fitting with threads that match your spark plug. Pressurize the cylinder to prevent the valve from falling (hope that your valves have a good seal. If they don't, then losing the valve in the cylinder and pulling the head is a good thing. You'll want to lap the valves anyways.) There is a setup (and the helm's manual doesn't show it, but if you do some searching around the net you can probably find it) that consists of a long bar that runs the length of the head. It bolts to the head and you use a lever that hooks under the bar and sits down on top of your valve spring retainer to compress the spring. Then it's a simple matter to put the keepers in place.

Taking the head off is easier, but you're going to have to order/fabricate some special tools to work with a standard valve spring compressor clamp. I used some square stock with keyways cut into it.