Sculpey is not good for this, it is also expensive.
first you want a clay that stays soft, a simple bulk oil-based non-hardening clay tends to work. I prefer softness #4 but there is a lot of different preferences depending on the material being molded, I was working with fossils where cleanup came second to preserving an extremely delicate original. If your original is tough or disposable go with a harder clay "medium" by most sellers, it makes clean up easier. You can heat it to soften it if it is too hard to work with or compromise and go with a "soft" clay. Some people mix their own clay with clay dust and grease but store bought non-hardening clay works fine, I tried it and never noticed much difference. You want something that is oil-based and non-hardening. if you heat it (and I recommend it) microwaves work OK as long as you get your timing down, don't melt it just get it warm, a toaster oven works better but tends to be kinda of small. I have an old bread oven but I do a LOT of molding.
Don't make your clay solid make ropes and build up to your surface leaving plenty of gaps, only the top half inch or so needs to be sealed, the rest of the clay should have gaps to allow it to be pealed away easily. I have also used lego or wood to build up to where the clay surface will be so only the top inch is clay. If using clay all the way through often I line the original with saranwrap or tin foil, so clay is only contacting the inch or so where the seam will be, makes clean up a lot easier. Just make sure it is not exposed or the silicone will wick down that material and waste a lot of silicon. (note if you use foil you don't want to use a microwave to heat your clay, I burned up a lot of microwaves before I got an old bakers oven to heat my clay.
using a release agent before putting the original in the clay will help, as will a simple dull dental pick for scraping away any left over clay. The cooler and harder the clay is the easier it tends to come off.