making a rubber mold

Hi All,
I am trying to make a "tool" to mold a rubber boot, I drew the part in 3d then tried to mirror it or reverse it, I dont know what the technical term is.
I planned on making the "mirror" image then coating it with rubber then turning it inside out I THINK(IDK) I'll have my part???
Am I on the right track I have attched 2 pics 1st is the part I want the 2nd is the toll I drew , will this work ?or How do you do it

thanks for any help, I know there are pros on here , LOL, I love this froum

or am I overthinbking this? do I just cover with rubber and dry and turn inside out?

I was thinking od using Ecoflex® 00-35 with black silc-pig, do I need re-inforcement material, the part is supposed to go up and down collapse and stretch out[/h]

I would assume that urethane rubber is more commonly used for rubber parts, but I have no experience with it. Maybe Beaker can help, he's the Smooth-On rep.