Install the tips and weld! The originals had flats to use a wrench to install them, but a pair of pliers or vice grips works fine.
The contact points of our Harbor Freight spot welder had gotten too pitted and damaged to work. I had no luck getting replacement parts from anywhere, so I made a replacement set. I used TechShop San Jose's lathe to make the parts.
Step 1: Load copper rod into lathe with collet closer
I bought a nice copper rod from McMaster-Carr - it wasn't cheap, but it was less costly than a new spot welder. The piece was 3/4" in diameter, so I used a 3/4" collet to hold the rod.