The rubber used is RTV, or Room Temperature Vulcanising, which simply means it sets without any pressure cooking. I'm using Micro Mark RTV rubber but the best source of RTV rubber in NZ is Topmark.
Different rubbers vary in their properties along a stiffness/strechyness spectrum. If you have a softer/strechier rubber, making the mold box larger to let more rubber mass lie around the master will ensure the mold holds itself in shape better when you're casting as the mold walls will be thicker and stiffer (preventing bent wagons), but this may make it harder to get larger castings out without tearing the mold. If your rubber is stiff, you may want thinner walls, but a large item (a 4w box wagon or larger) with thin rubber walls will need support against the weight of the resin or you’ll end up with bulging sides in your castings. You'll figure out pretty quickly what works for the rubber you have. Stretchier rubber tends to last longer than the tougher stuff which tears easier.
(Here at the Waihaorunga Creek workshops we are using the blue stuff which also goes by the name Ultrasil®. It appears to be a stretchy type rubber which has not given me any trouble yet)
Once you thoroughly mix up your rubber and let it settle briefly to remove air bubbles, the key to getting a good mold free of air bubbles is to brush a thin layer of rubber all over the master with a paintbrush (and not your Sunday best paintbrush that you save for special occasions either), ensuring you get a thin coating in all the nooks and crannies. Once this is done you can carefully pour the rubber on top slowly. Once you’re done, it is common practice to slap the mold box on the table a few times to dislodge any remaining air and scare away the air bubble spirits.
(At this point I tend to spend 10-15 minutes slapping my rubber. Its to get the air bubbles out, honest!)
Give the rubber plenty of time to cure per the instructions that you probably threw away without reading them, and then carefully remove the set flubby (blue in my case) blob. Magic isn’t it? Sometimes destroying the mold box is a good idea so that you can flex the rubber and carefully extract your master. Hang onto that precious master as you might want to make a second mold later - either to speed mass production, or as a replacement - all molds fail and tear eventually.
You should be able to get your master out without tearing the rubber mold, if not, you will struggle to get the castings out as well! There will probably be a thin meniscus where the rubber meets the mold box and in the interests of getting a nice flat mold bottom, you can carefully cut this off with scissors as a final step.