Step 2: Take apart the old base
Step 3: Make a hole in the back of the base
Step 4: Pass the neck of the lamp through the bottom of the pot
Stab the neck of the lamp into the soil and out through the hole at the bottom.
Then, take a drill and make a hole in the bottom of the plant saucer. If it's ceramic, you can still drill through it, just go slowly, don't push too hard, and put some water on the drill bit as it works its way through. Mine happened to be made of galvanized steel, so it was easy to drill through.