Referring to Angel Trumpet, do you mean Datura or Brugmansia? Both should be similar in regards to collecting. They will develop a pod, once the pod splits, then you can remove it and collect the seeds within....let them dry.
Certain Coreopsis are sterile, if they are not, let the flower heads completely dry on the plant before collecting...should be a little brown ball at the end of the stem...
Pick these off into a container then go to disecting them...the seed will look a little like a tick...usually dark brown to black in the center (kind of like a cat's eye) with brown surrounding them...