2 equal lengths of coat hanger thickness wire 90 degree bend in each a the ends to make a short handle.
2 short bits of tub to accommodate the handle this is to allow the wire to move freely.
hold in your hands about 300mm apart angled slightly down.
Go for a slow walk, if the wire moves as if to cross each other then you have the knack change your angle of attack to establish if you are following or you have a single spot target.
The thing is you may be detecting water, static or flowing flowing gives more reaction, you may also detect electrical cables.
30 odd years ago I thought this was a load of old tosh, until I was shown this by the agronomist from SAC during a visit when I mentioned a lost water pipe to a field trough.
If you've "got it " after practice you can learn the differences.
Good luck