Datura alkaloid extraction is fairly simple, simply soak the plant matter in methyl or ethyl alcohol, the alcohol will turn a translucent yellow, allow the plant matter to soak for 24-48 hours, filter well, discard plant matter, evaporate the solvent, a golden/yellow powder residue will remain. (If using Methyl alcohol, be sure it is fully evaporated due to toxicity concerns.
Datura species (as well as other plant species), contain tropane alkaloids such as scopolamine, hyoscyamine, and atropine, these alkaloids are primarily muscarinic receptor antagonist compounds, their action involves Acetylcholine inhibition. These Acetylcholine inhibitor compounds induce an unpleasant state of delirium, which is accompanied by various manner of physical malaise and distress...
I'm not entirely certain that these are compounds which one would necessarily want to extract, the plant matter itself is toxic enough where the amounts needed to be consumed to induce intoxication are fairly minimal...
(Be sure you understand the nature of tropane alkaloid intoxication before attempting to ingest these compounds...)
As for Calendula, these have been used to treat many minor ailments, stomach cramps, skin problems, tooth ache, etc...
The flowers of C. officinalis contain flavonol glycosides, triterpene oligoglycosides, oleanane-type triterpene glycosides, saponins, and a sesquiterpene glucoside -Wikipedia
Is there a specific compound, with a specific purpose you have in mind regarding Calendula extraction?
-eg