Step 2: Form spider abdomen.
First, a few warnings to protect yourself and your work environment. This step requires extreme caution and adult supervision, if applicable, as it involves molten plastic and open flame. Wear gloves and use forceps or pliers to keep the hot plastic away from your skin.
Always allow the cord to cool after heating - the plastic can appear to be solidified, but will remain in molten liquid form for up to one minute after removal from heat.
Complete this step in a ventilated area - fumes from burning plastics can cause lung irritation and dizziness if inhaled.
Protect your working surface - drips of molten 550 cord will permanently damage carpet, unfinished and finished wood, and fabrics.
With those caveats out of the way... the construction process begins. This is the trickiest part of the spider construction, as the size and appearance of the abdomen will dictate how the rest of the spider looks.
For extremely small spiders, I recommend "gutting" the 550 cord - remove the inner strands and work with only the outer sheathing material. This will help reduce the size of your finished product.
Begin by slowly heating one of the ends of a length of 550 cord. Rotate the cord, avoiding drips, until you have accumulated a small blob of melted plastic about the size of a pencil eraser. Allow this to cool, and then set it on your working surface. I used forceps to secure the cord and suspended it over the edge of my work table (as pictured in the 3rd photo below).
Take a second short length of 550 cord and begin to melt it. Allow it to ignite, and then, with a stroking motion, "paint" additional layers of smooth plastic over the original blob. Each time the burning "paintbrush" is applied, the flame will extinguish, and you will need to melt and ignite it again.
Once the desired size and smoothness is achieved, the abdomen is completed. In the accompanying images, I stopped once my spider abdomen reached the size of a coffee bean.