lets see how to explain it the easiest way possible.
First step is looking for a card in bepis web, generaly the keywords for tags should be Custom_modules or Custom_triggers.
After downloading the card youre interested open the card in the editor and go to modules tab. (for reference im gonna use a card that i really apreciate and attach some screnshoots) as you can see this card for example has the messenger of love module, a module that basically is not in the base modules that came with the game. in this step as you said you unpack the modules.
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The second step is going to the triggers tab, here you will find the unpacked module, this step is a bit tricky cause some cards has some random unamed triggers, so you should note beforehand unpacking the module what is that was unpacked in the module, after making really sure what are exactly the things that where in the module then you select them and press R mouse button and select "export as module" (again i attach a screenshot for reference)
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After that you should name the module and put a description, some modules are a little more complex that others and have dependencies of other things but lets put that for another time, cause almost every module on cards are not that complex, well going back to the point, after naming the module and writing the description it should appear in the modules tab as any other normal module, i recommend to test them cause sometimes you can pack something unrelated when exporting the module and things will not work as intended.