Say you are giving signs to catcher from dugout: Make up a signal for a pick to 1B. Can be as simple as if I finish series of signals on the hat, I want a pick to 1B. Catcher then puts hand down to give sign & instead of 1,2 etc, he flips his thumb towards 1B (subtle, don't jerk the thumb). Pitcher then knows to pick 1B. (Runner & opposing coaches unable to see this thumb flip)
Second base Pickoff. Again, make up whatever you want. Finish on chest = pick to 2B. Catcher puts signs down: puts down 2, 2, 2 then a fist. SS has to be looking in to see it. Pitcher & SS then know pick 2B. Pitcher comes set & continues looking HOME (NOT TO 2B!) SS breaks & when catcher sees daylight between SS & Runner he drops his glove, this is the trigger for the pitcher to spin & throw to 2B. ** This is not an "inside move to 2B," but a quick spin & throw. You will get more than you would expect.
Don't pick to 3B. Usually a disaster & where is he going anyway.....