Trying to remember the mistakes I made when I first started out!. Here's a list of basics that I was able to come up with:

If the model you have has a meter, lay your existing earring on the ground, then see what the meter reads when you pass the head over it. This is the sort of signal that you'll be looking for. While you're at it, note also how far away you can be from the earring before the signal peters out. Since depth is close to the same in the air, the ground, or snow, this will tell you if the snow may be too deep to find your earring, in which case you might be better off waiting until it's melted. Search systematically, swinging the head in parallel, overlapping strokes. Don't lift the head at the end of the stroke, and hold it parallel to the ground at all times.

Keep the head as close to the ground as you can - for something as small as an earring, your depth will probably only be a few inches, so if you're holding the head 3" above the surface, it might not register at all.

"Ground Balance" your machine before you start. On most White's models, you do this by pushing "enter" with the head on the ground, waiting for the beep, then pushing "enter" again with the head at waist level.

When you get a signal, dig out handfuls of snow from the place where you got the signal and wave each of them over the detector head (use a hand with no rings or watches!). If you get a beep, you know that you have the target object in your hand. Divide the snowball and test the pieces to further localize the target until you can pick it out of the snow and identify it.

The ubiquitous pull tabs and foil will give a signal that is about the same as that of gold, so be prepared to find a lot of aluminum trash in addition to your earring.

Above all, if you can find somebody with experience using a detector, ask them to help. There's nothing particularly mysterious about using a machine, but an experienced detector operator may be able to accomplish in a few minutes what might take a novice several days.