Probably one of the harder processes of making a mold. Orientation doesn't matter so much with smush molds, but with Sprue Molds its very important.
Basic science says Gravity works with the path of least resistance. When poured resin will follow this rule. Gravity will pull it down and a path through the silicone mold will guide it. The resin flow will push the air out the vent paths.
But how do you orient you master? The rule I always try to follow is Point It Down. This is aided by our 2 rules. You never want your resin to have to flow up into an enclosed space. This will create an air pocket. The air is trapped with no place to go. So your resin flow should always be down.
Sometimes this is unavoidable because of the shape of your master. If putting in a vent is not a viable choice, you can modify the mold. Just cut a little bit of the mold to create a pocket for the air to escape into.
A vent is a smaller channel that simply allows air to escape. This will fill with resin so you don't need very big channel and you will clip these parts off and throw them away or keep them to use as channels for other molds. A toothpick will make an excellent vent minimizing the wasted resin that will fill it.
The more complex your sprue mold, the harder it will be for the resin to travel through the mold. I would recommend a resin with a longer pot life to ensure it doesn't start to cure while you are still pouring.
When pouring the resin, we are fighting the properties of Adhesion. Adhesion is what keeps a drop from going completely flat on a surface. The water adheres to the surface, keeping the water drop from covering more surface area. This is the same with resin.
Conversely...Sometimes before you pour your resin, you might want to apply a little bit of resin to hard to reach areas before you pour. When the resin hits this area its will use the properties of Cohesion, attaching itself to a liquid of similar properties to help it advance through the mold channels eliminating the chance an area wont get covered with resin and leave you with an air pocket.
I'll update this soon with some pictures.