Issues with cargo bays to drop satellites and rovers

[paul conto]

Trying to use carbo bays on space planes to drop satellites into orbit and drop rovers onto planets. MY issue I'm running into is how do i put the rover or satellite into the cargo bay. So far the closest I've gotten is "attaching" the item to the cargobay front or rear and build the craft around it. But then i can't drop the cargo. It's really frustrating see videos of people doing it but i can't find ANY tutorials or tips on how to do it. I know its best to build them separately then combine them but how?? Thank you

[Pecan]

Use a decoupler or docking port. Attach it to the cargo bay and then the payload to the decoupler/port.

[Starwhip]

Yeah, you're missing a decoupler. Fairly easy mistake, too, I've forgotten decouplers on my stages sometimes. Landing is not pretty at that point.