Insert a filling knife into the crack. Move the knife back and forth until the crack becomes a "V". You want the wider portion of the V to be inside the rendering.
Use a small wire brush to remove any debris from inside the newly formed crack.
Wet a sponge and run it over the crack. Moisture will help the filler stick.
Push rendering filler into the V-shaped cracks using your filling knife. Make sure you push the filler all the way into the crack so that it gets into the bottom of the V.
Scrape the filling knife across the surface when you're finished so that the filler will dry evenly.