Not to have free will would feel like when I am not conscious of the fact that free will exists. I would actually operate the same way as usual. Free will is just an idea that appears when I think about determinism and randomness in the universe. Also, I think about free will when deconstructing the universe and trying to understand how it works. This is because as a way to compare "dead" physical mechanics to "non-dead" I use as a reference the supposed free will I have in my mind (or the feeling of free will).

Summary: Free will is the name I designate to a group of activities in my mind that result in a decision. But it's not a thing or something that actually exists anywhere, but in my imagination!