It would be far more effective to just demonstrate in person, but basically you go straight from the "ng" sound to a "v" sound. The "(ha)ve" is a weak syllable, so there's no stress on the pronunciation at all.

"long've" = “long love”

I can't access the phonetic alphabet, but the " 've " (contracted have) is a schwa sound (unaccented syllable - like an upside-down 'e') and a 'v' sound. It sounds similar to the word 'of' in a phrase like 'cup of tea', but even shorter. Some people may even pronounce the " 've " as it if were not a separate syllable at all ie with the 'ng' sound sliding straight into the 'v' sound. Hope that helps.