I don't have a drawn schematics. There must be dimmer schematics on the Internet.
You can look at the outocouplers, such as
this one. The data sheet has typical schematics with the optocoupler and the triac connected to it. You need a big triac, such as BTA24, and a big heat sink.
You will also need some circuit that will need to drive it, which is not straightforward because you need to detect zero crossing and drive triac at the specific time in the AC cycle, which IMHO is best done with a microcontroller.
To simplify, you can switch the triac on and off once a second or so, which doesn't need zero crossing detection and doesn't need microcontroller, but can be controlled by 555 timer running at 1Hz rate. However, it's not a very small project either.
An even simpler way would be to buy two heating elements and connect them in series. This will produce 50% reduction of power compared to the single element.