Go ahead and remove the valve covers and all the spark plugs. You're going to have to do that, anyway.
The cylinder will be on the compression stroke when both valves are closed. Stick a (preferably) wooden dowel into the cylinder, and let it rest on top of the piston. Carefully turn the crank pulley. You will be able to feel when the piston is at (or approaching) the top of its travel, by feeling what the dowel is doing. Be careful not to let the dowel get bound up, and break off. The cord that you're feeding into the cylinder is to stop the valves from dropping down onto the piston, or into the cylinder. (If you fed in 11', the piston is still way down in the cylinder.)
An alternative is available if you have an air compressor and a sparkplug adapter. (If you have a compression gauge, the "screw-in" adapter should work fine.)
It doesn't really matter where the piston is on its stroke.
Remove all of the rockers. Mark where they came from. They need to go back in the same place.
Connect the compressor to the cylinder, and pressurize it. Full pressure is fine. You generate much more than that, every time the spark plug fires.
Once the cylinder is pressurized, you can do all the work you need to do to replace the seals. The air pressure will hold the valves closed.
It really would be a good idea to have the piston at the top of its travel, in the cylinder that you're working on. If the pressure drops, for whatever reason, the valve still shouldn't drop into the engine.
If anyone sees any flaws with this method, please comment. It's what I was planning to do when I replace my valve springs.