This was already an old-timer's trick when I first heard about it a long time ago.
There could be several reasons why you may want to remove valve springs with the head(s) installed on the engine; replace a broken valve spring, replacing valve seals or installation of a performance cam with a break-in procedure that requires initially running the engine without an inner valve spring installed to name a few.
Step 1: Keep the Valves in Place
Normally, to remove the valve springs without removing the cylinder head the sparkplug for the cylinder you are working on Is removed, an adaptor is threaded into the sparkplug hole and a continuous supply compressed air is introduced into the cylinder through the adaptor (some air will leak past the rings hence the need for a continuous supply of air). If no compressor is available than some other means is needed to keep the valves from falling into the cylinder.
Step 2: A Few Simple Tools and Some Rope
All that is needed is a hammer, ratchet & sockets, and a valve spring compressor designed for use with the head installed (about $20 US). A small screwdriver and magnetic pickup will also help.
Step 3: You Will Still Work Through the Sparkplug Hole
The sparkplug needs to be removed to gain access to the cylinder, use care as the porcelain end of the sparkplug is relatively fragile and is easily broken if the socket doesn't stay straight on the plug (specialty sockets are available that minimize this possibility).
Step 4: Rotate the Crankshaft Puttting the Piston Near the Bottom of the Cylinder
Rotate the engine manually (one way is to use a socket on the bolt in the front balancer/pulley), continue to rotate until the piston is at, or near, BDC (Bottom Dead Center). You may be able to observe the piston through the sparkplug hole, if not, place a plastic straw or some similarly pliable probe down the sparkplug hole until it touches the top of the piston and rotate the crankshaft "feeling" where the piston is at the lowest point (make sure the probe is long enough not to fall into the cylinder).
Step 5: Remove the Rocker Arms
Remove the rocker arms, this will ensure the valves are closed (unless you are doing this due to a broken valve spring, more on that later). Using the bottom of a hammer handle tap the valve spring retainer to break the fit between the retainer and keeper (see image for parts identification).
Step 6: Fill Some of the Space Above the Piston
Using rope made of a something that won't shed any fibers, feed the rope into the cylinder through the sparkplug hole. Continue to feed the rope into the cylinder, loosely filling as much of the cylinder as possible.
Step 7: Rotate the Engine
Again, rotate the engine, by hand, until you feel resistance (the rope pressed between the piston and valve). If you have a broken valve spring and the valve does not close completely keep some upward pressure on the valve so the rope does not get caught between the valve and seat.
Step 8: The Valves Will Be Held in Place by the Rope
Use a valve spring compressor, designed for use with the head installed, to compress the valve spring, remove the keepers (a small magnetic pickup will help) and the spring. Complete whatever repairs are needed.
Step 9: Removing the Rope
Rotate the engine in the opposite direction, relieving the pressure between the rope and valves, remove the rope, and you are done.