When the head is moving across as it checks paper size, open the front cover. The head will stop. Turn the printer off. Then you can gently slide the head assembly to the left and get access to the head cap columns. I've never done anything other than dab them gently with clean wet foam. Close the door and start it up. The printer safely returns the head to its home position as part of the boot sequence.
A technician had to replace a print head because of some bad nozzles, then several weeks later he had to replace the entire head cap assembly because it wasn't sealing properly. Through all of this, I got to study the printer's guts, how cleanings work, etc. I watched him shoot water into the head cap assembly with a syringe to moisten it and to prime the pump that sends the ink into the maintenance tank. That was an epiphany for me, especially since our dry winter days have historically resulted in major nozzle problems. Not any more though. :-)