I remember an interview from Swann where he advised spinners start off with a fielder in the cow corner as a protection.When your confidence,flight,turn and correct pace for the pitch is found, you can bring that fielder in dependent on the batsman. Other things to consider is your line of attack dependent on the spin, obviously you would want to entice the batsman to play against the spin and hence put fielders accordingly. Having a staggered field set rather than fielders in line with each other can help to confuse and hesitate singles. I hope this makes sense.