Perhaps it is easier to provide an example of using a standard SSR to control the temperature.
What has to be done is the on / off time must be multiples or 60Hz or 50Hz in some countries. I'll assume 50 Hz for this suggestion.
define ticks as 1/50 (aka 20 ms)
Your control must be in multiple of "ticks"
i.e.
1 ticks on then 1 ticks off (50% power)
2 ticks on then 1 tick off (2/3 power)
3 ticks on then 2 ticks off ( 3/5 power)
etc