In British English we call the reduced version of words 'the weak form'. (Not all words have weak forms, but a lot of common ones do, and most words of 2 or more syllables have weak syllables). If you google 'weak form in English' you should find some videos, as these are more helpful that having pronunciation written down. However I'll write it here, and it may help. Should have......can be pronounced should've or (even more commonly) shoulda in phonemes /ʃʊdə/ Could have.......could've or coulda in phonemes /kʊdə/ would have......would've or woulda in phonemes /wʊdə/

Any way that your listener understands is good :) These are often condensed a bit, in what is sometimes called "relaxed pronunciation". If you can bear it, it might help you to look at these two links :) https://en.wikipedia.org/wiki/Relaxed_pronunciation#English https://en.wikipedia.org/wiki/Pro-drop_language#English