Step 1: Tie a double overhand knot (two wraps instead of one) in the end of the dropper line. This will form a non-slip knot at the end of the dropper line. Trim the tag, but not too close to the knot.
Step 2: Form a loop in the leader where you want the dropper attached.
Step 3-4: Pull the standing line of the leader through the loop you just created, forming a loop through which you pass the dropper line with the knot.
Step 5: Pull both ends of the leader to tighten on the dropper line.