What you'll need is several feet of single conductor, solid core electrical wire of a gauge stiff enough to hold a section of your decorations.
A pair of wire cutters to cut sections for fashioning the hangers is the only tool needed.
Step 1: Select Your Wire
I got my single strand of insulated electrical wire from a length of 14 gauge three conductor Romex left over from a previous project. I slit the outer casing to separate the white, black, and uninsulated copper ground wires. My railing has white spindles, so I chose to use the white insulated wire for my hangers.
Step 2: Fashioning a Hanger
Take one end of the long hanger wire and place it pointing down behind the rail. Bend the wire under the rail, forward past a spindle or post, across the front of the spindle and back along the other side off the spindle.
Now bend the long wire coming up from behind the railing so that it crosses over the top of the railing and then bends towards the ground.
Cut the long wire at a point an inch or so lower than where you want the decoration to hang. Now bend the end of the wire up to form a small hook.
That's it. You've made one hanger. Repeat as needed.
Step 3: Handling Corners
Step 4: Using, Removing, and Reinstalling
To remove the hangers, remove the decoration and then tip the hook up towards the railing. This will tilt the partial loop that is around the spindle or post. Twist the tilted hanger to the side to release the partial loop from around the post. The hanger can now be lifted from the railing and stored until it's needed again.
To reinstall, reverse the removal maneuver. Put the spindle loop behind the railing between two spindles. Angle the loop in front of and around a spindle. Pull the hook down in front of the railing.