General Question Rocket's carrying cargo: How to?

[Kaito]

So, I have some rocket addons, but all they are is just the stages. Is there a way I can get it to carry a custom payload? (by custom, i mean i select one for it to carry)

 

[garyw]

Yes.
The method varies from rocket to rocket though so more details are needed.

 

[Arrowstar]

Velcro seems to work fine, as does Kulch's UCD.