Sculpting is the hardest to do with minifigs because
the minifigure is small so if you want detail it is quite hard
to get the details incorporated into the sculpt of your
accessory. (Note: if you are ready had experienced sculpting
with much detail this will be not as hard.)
First thing you want to do for the sculpt is get a unwanted
minifig head and torso (for helmets and such) or for a utensil
or weapon a arm and a hand attached to a torso on a base plate.
Get the example of the accessory your sculpting. Next
get the clay soft enough to mold (note:if the clay is old it will be harder to soften but keep doing it!)
(note: you could always base your sculpt on an existing accessory so its easier to make!)
When sculpting add kind of thicker layers on the sculpt because you will probably do a lot of sanding
to get the part looking good.
Once you have your initial sculpt done for your minifig get that Tupperware container mentioned earlier
and fill up water in it until you get enough water to submerse your sculpt in it.
After that put the container with water in the microwave and heat it until boiling then put your sculpt in there softly.
(NOTE: use a oven baked clay for this!)
Watch the sculpt closely and bring it out of the water a couple of times until it has cured.
(note: Leave the part out to cool for at least a hour until working with it)
Once you have sanded it I would recommend using primer on
it a then sand softly and repeat that step
a couple of times to get desired smoothness.
Like I said back in tips and tricks, Revise your sculpt many times until you like it.
My Flickr page has some of my sculpts for you to see