Where to hide your bad fielders

In swing: cover point or third man.
Out swing: mid on, midwicket or fine leg.
Off spin: short third man, cover point or square leg (saving one).
Slow Left arm or Leg spin: Midwicket or square leg (saving one).
No swing/movement: Slips.

The principle is pretty simple: Put the fielders in places that are against the natural movement of the ball because its very tricky for the batters to hit it there.

NB. Really confident captains can also use bad fielders in a positive way. For example, if you are trying to get your opponents back in the game enough for them to go for it and get bowled out you use the bad fielders to quietly leak runs. Sneaky.