Well to cast it you will need to plug and/or seal up all holes, seam lines and cracks... The silicone will seep into every crack and crevice... You might also want to clay up the trigger to the trigger guard so that the trigger isn't just hanging there, connecting the bottom of the trigger to the trigger guard will help the flow of resin and also strengthen the trigger area... Then you need to decide on a parting line(s) clay it up and start molding it paying attention to the fill location and any other vents you need to avoid air traps...
There isn't one right answer to how to mold it, a lot of it is simply experience or lots of thought process, or simple trial and error...
Being a revolver it might be beneficial to cast it in multiple parts, especially the cylinder, you will need to decide what route is best...
If you have no casting experience I suggest you practice on something smaller and get a feel for the entire process...