What product would be best to use for attaching it to my face? I do not want it to be ruined, as I worked really hard on it and it took me about 2 days to complete.
The clay is by Crayola and is somewhat soft (?) when dried.
Spirit gum is the classic adhesive for makeup work. This time of year you can get it wherever Halloween costumes are sold. The rest of the time, amazon probably carries it.
As for improving your creation's durability, I would suggest painting it with several layers of skin-friendly non-oil-based makeup paint. Allow this paint to thoroughly dry before attempting to glue it on.
That material is not likely to survive very long. Normally you would make the piece out of latex or silicone, because those materials are resilient. However, I can recommend that you use rubber cement. It is generally (unless you are allergic, which you can test by putting a small dab on your skin, if it itches or get red, wash it off immediately with warm soapy water). Assuming you are not allergic, now try to attach another sample as a test to your face. Figure out how much glue and how strong the clay is. Do this by applying the cement to the back of the clay and immediately attach it to your face.
If it works, then do it with the final piece. Good luck
My first choice would be spirit gum. Spirit Gum is an industry standard adhesive, and you can easily find it online. I personally recommend the brand Ben Nye. They are a credible company that a lot of celebrity makeup artists use.
However, if you are unable to obtain spirit gum you can use eyelash glue. Eyelash glue is typically use for adhering fake eyelashes to make them appear bigger and darker. This you can also easily find online. Eye lash glue is relatively safer for the eye area and more accessible. That being said, always do a small patch test on your hand prior to your application to ensure you don't get an allergic reaction.