Howdy fellow Iowan. Hard to believe you built that panel without understanding how SSRs function. But, looks nice.
One side of the SSR should be marked "input" with a +/- and a voltage rating, something like 3-36v VDC. This is where you connect the PID. These need to be connected as indicated by the polarity. Be sure your PID is set to control and SSR, as opposed to a mechanical relay.
The other side is where you connect the power for your heating element. This will be only one leg on a 220V element. That side will be marked something like ~24-~380 VAC. This side has no polarity, it simply acts a switch, use one pin as power in and the other as power out.
When the PID provides power to the input side, the circuit closes and power is allowed through.
There are a terrible lot of lies going around the world, and the worst of it is half of them are true.