A bit hard to explain from here, but here's the basic premise:
1) Disconnect power
2) Disconnect ALL devices on that circuit (e.g. remove them from the boxes
and remove the wires). MAKE SURE you remember which wire goes where -- that
is -- label everything! Also make sure nothing is touching anything (that
is, bare wires pointing into air and touching no other wires or the box).
3) Turn on power. Verify breaker doesn't trip (If it does, you either
missed a device, or the problem is between the breaker and the first device.
4) Using a neon tester or multimeter, find the one HOT wire
5) Disconnect power.
6) Reconnect that device.
7) Reinstate power.
7a) Breaker trips? You've found the bad device (or the wire is bad between
this device and the next in the chain).
7b) Breaker doesn't trip? This device is good. Go to step #4.
Hope that helps.