Start by training to the whistle - if they already sit, then introduce a single beep so they sit on command, if not, train for the instant sit. If they already recall instantly to name, then introduce 3 beeps - if they are not instant then train for that first, then take the whistle in. Then introduce a turn whistle - 2 beeps. Have the dog on a line, run to the right, whistle 2 beeps, when the dogs runs past you, turn and run in the opposite direction and blow 2 beeps again, and do this over and over till you get to the end of the field - it's fun!
Then teach them left and right on verbal commands. Use some of their food - they will follow your arm movement - make it obvious!
What that will then mean is that they will run out to whatever direction you say - right or left, away or come bye - go or stop - use any words you like but be consistent. They will stop when you tell them to. Then you can tell them to go right or left, then you can recall them - complete with a couple of hundred sheep or 4
Whether the sheep will do as they are told is a different matter