You first need to know what type of modelling clay you are going to work with. This will help you understand whether you just need broad day sunlight, or an oven to make the modelling clay hard. Moreover, different modelling clays have different hardening techniques; you might be comfortable with one and not the other.
If the clay you are using is Fimo, a brand which contains polymer, then you need to bake whatever you have created. In order to carry out this process you first need to preheat your oven to 120 degrees Fahrenheit.
After that, grab hold of a wax-lined cookie sheet and place your object on top of it. Make sure that the object you have made fits the cookie sheet, in case it doesn’t, you might need more than one sheets.
After you have inserted the cookie sheet inside the oven, simply wait for 10 to 20 minutes, depending upon the size of your creation.
If the clay you have purchased is oven-hardening or non-hardening, then you will have to bake it for a longer time. Follow the same steps all over again, but in this case you have to put your object inside the oven for at least an hour, with the temperature set at 300 degrees Fahrenheit.
If you do not want to use ovens, you may use a kiln. In order to pull this technique off, you must let your creation dry off for at least a week and then take it to the nearest kiln.
In case you have self-hardening clay, you will have to wait the most. It takes almost a week, or even more, to dry off at room temperature.