Our first year, elf Ollie showed up under our Christmas tree over night around December 1st (he now shows up the day after Thanksgiving). He was just sitting there (and we didn't say anything, we just let the kids discover him) with a letter from Santa. Basically the letter stated that Ollie was there to watch them and report back to Santa their behavior. It also stated that the elves have been known to get into mischief and to look out for any elf mischief. It explained that he would be staying until Dec 24 when Santa would come pick him up during his "rounds".
I don't know if you plan to have your elf get into "mischief" or not, but if so, there's tons of great ideas on this board. Some people like their elf to just move around the house and not really cause "problems". Ollie.... he gets into all sorts of trouble including using a zip-line in the house, having big parties with streamers and balloons, decorating the house with toilet paper, hanging the kids underwear on the tree as ornaments and even stealing the car for a late night run to McD's. The kids love it! It's a great holiday tradition IMO and one I hope your family enjoys too! Good luck!