All I do is poke the cuttings into potting mix in 4-inch square pots, water completely, then put the whole tray of pots into a big clear plastic tub and place the lid on loosely. Put it in the shade where it can't get any direct sunlight (it's basically a mini greenhouse, so it'll get really hot inside if the sun hits it). Check it for moisture levels frequently to make sure it doesn't dry out, and they will probably be rooted in a month or so.

Rooting hormone helps, but isn't strictly necessary for elderberry.

Unless you're in a real hurry to replicate the plants, you'll probably have better luck if you wait until it's a little cooler, but you can still root them in summer.