Place your shallow bowl in the center of the larger platter, then spoon your guacamole in and smooth the top with an offset spatula. You want a flat, even surface so the crema sits nicely on top without sinking in unevenly.
Fill your squeeze bottle or pastry bag with the crema, thinning it with a splash of lime juice if it feels too stiff to flow. Pipe four even, concentric circles onto the guacamole, starting from the center and working your way outward.
Take your skewer or the tip of a paring knife and gently pull it from the center circle straight out to the edge of the bowl. Repeat this evenly around the whole circle, and you'll watch a spider web pattern appear almost instantly.
Slice one olive in half lengthwise to form the spider's body, then cut eight thin strips from the remaining olive for the legs. Arrange them right in the center of your web so it looks like the spider is sitting proudly in its trap.
Using a paring knife, cut two eye sockets into the wide end of the avocado, slicing through the skin down to the pit. Shape a small upside-down heart for the nose and a rectangle for the mouth, then score a few teeth lines into the exposed flesh.
Set the finished skull beside your guacamole bowl, then surround everything with your tortilla chips. Serve immediately so the avocado skull stays fresh and green for as long as possible.