Outcome

This drawing shows where I planned to put the parting line in the mold. 

I started by packing clay around the model of my grip.

Once this was done, I smoothed out the clay using tools and cut it to fit in the box. I was careful to make sure that there was a well defined edge where the clay met the model.

I then built a box around the part to pour the silicone into. I made sure that the clay sealed well to the edges of the box to prevent leaks.

This is the box after pouring silicone. For the second half of the mold, I removed the clay and then built the box around the bottom half of the mold. While removing the clay, I slightly damaged the model of my part. I made the box slightly smaller than the mold in order to compress the silicone and ensure a good seal around the edges of the box and between the mold and the clay part.

This is the completed mold after removing it from the box.

To prepare for casting, I cut a pour spot into the end of the mold, and secured both halves together using rubber bands.

On my first casting, I accidentally stuck the funnel too far into the mold, creating a hole in the part. For the next cast, I cut the end off of the funnel and marked how far I should put it into the mold.

Despite the issue with the funnel, the finish on the rest of the first cast was very good.

With the second cast, I think had the ratios for the resin slightly off. The part had many small bubbles all over the surface, which are visible in the image above. The part was also somewhat oily.

My final casting did not have these issues, and I chose to do some finishing work on it in order to make it neater. I used a little bit of waste resin to try to fill in the air bubbles near the pour spout. 

I started by cutting off the pour spout and grinding it down with a dremel. The air bubbles are still visible despite the coating of extra resin. If I was making another casting, I would cut a couple vent holes near the pour spout to help air escape. I tried to rotate the mold to help the air escape, but that was clearly not enough.

I then ground off the parting line. I finished the part by sanding with 200 grit and then 1000 grit sandpaper. I discovered that using such a large gap in types of sandpaper isn't a very good idea, since the part still has some visible scratches in it. However, it still feels very smooth.

Final casting