If you are taking a cutting from an elderberry plant you found while foraging, you should mark the location of the plant so you can come back and get your cutting over the wintertime for the best results.
Choose a cutting that is at least 6 inches long from a healthy, yet dormant elderberry branch. Look for one with at least two node pairs as these assist in rooting.
Elderberry needs to cross-pollinate. This means you need at least two elderberry bushes preferably of different varieties and keep your plants relatively close.