I recommend using a small paring knife to slice the olives. This will be much easier than trying to use a big knife for the little details. First, cut the olive in half and set one half aside to be the body of the spider. For the other half, we want to cut it into eight slices to make the legs.  I think the easiest method is to cut the olive across the middle width-wise. Next, cut each side in half again so you have four pieces. Finally, cut each piece in half once more. Now you have all eight spider legs!