Very simple, connect a pid controller to the relay, connect the relay output to the contactors coil on one phase and have a constant supply to the other end of the coil.
Connect the elements through the contactors contacts and you are done.
Normally would add a on/off switch to the setup to, your sockets don't like being unplugged with a electrical load on them.
Without knowing your relay, PID controller and Contactor the diagram is hard to make accurately as there are a lot of options.
When you have your components that are compatible, feel free to contact me, if i get the specs I should be able to make a diagram in 5 minutes and scan it.