RE: white tail deer decoy...when...how
I have limited experience with decoying for deer. I have used one on several occasions but always with mixed results. I, obviously, have not gotten it down to the science that many others have.
Some basic suggestions would be to keep the decoy outside for a few days before placing it in the field to get rid of much of the human scent. Also spraying it with odor eliminator and wearing gloves when setting it up is normally recommended. Some folks also attach a small white rag to the tail area. This serves two purposes...One, you get some movement if the breeze picks up. That makes the decoy look more realistic. Two, you can apply some scent to it so that, again, it appears more realistic.
Most folks also recommend that you face the decoy broadside from your position so that when a deer comes to either mount it or fight with it then the deer will also be in a broadside position.