I pressure ferment, so I transfer carbonated beer keg-to-keg all the time. The trick is to have your corny just below the same PSI as the sanke, that way the beer doesn't de-carbonate due to the pressure difference. Of course, you don't need to worry about it if the beer in the sanke is already flat. I use a spunding valve to keep the corny at the pressure I need while the transfer happens, but I'd imagine you can do the same thing just by pulling the PRV manually every few seconds.
So:
1. Start with cold beer, if possible. The cold helps keep the CO2 in the beer, and it also helps you figure out when to stop and not overflow the corny.
2. Build a jumper: that's a hose with the sanke tap on one end and a corny disconnect on the other. You need to reverse the direction on the corny -- beer goes in the "out" and (if using a spunding valve) gas will come out the "in". And using clear hose for the jumper helps you keep an eye on things.
3. Pre-pressurize the corny to a couple PSI below what the sanke is.
4. Connect the corny end of the jumper first, and then the sanke.
5. Adjust the pressure difference between the sanke and the corny so the beer flows nice and slow.
6. If you don't have a spunding valve, pull the corny PRV (or push the gas poppet) when the beer gets too slow or stops. Try to only relive enough pressure to get the beer to flow slowly again.
7. If the beer is cold, watch the condensation line (or feel the temp with your hand) on the corny to help know when to stop. Otherwise, good luck guessing, and try not to make a mess.
8. To stop the flow fast, pull the corny end of the jumper first. The disconnect should stop the flow.
Obviously, the corny needs to be cleaned and sanitized first.
[Edit] Just re-read the OP, and realized you're using a hand tap for the sanke. That might complicate the process a bit. If the beer is already flat (and/or you don't mind re-carbonating), just pour the beer into the corny like it's a big growler. Try to stick some tubing on the end of the faucet to get the beer to flow into the corny as low as possible - you want the inlet to be below the "water line" as quickly as possible.
