Say it FER-vuhnt-lee, three syllables, stress on the first. UK /ˈfɜː.vənt.li/, US /ˈfɝː.vənt.li/.
The varieties differ only in the first vowel: US curls the tongue back for an r-coloured vowel, UK leaves it long without r. Both keep the last two syllables light and quick.
How your mouth makes the word
Keep your tongue central and relaxed for the first vowel, held a little long. For the US form, let the tip curl slightly back for the r-coloured vowel. Say FER with strong stress.
Let your mouth relax for the second syllable. Say a quick, unstressed vuh, with your top teeth touching your bottom lip for the v.
Finish with a soft nt-lee: tongue tip taps behind your top teeth for the n and t, then a light l before ee. Keep those last syllables quiet and quick.
Say it in a phrase, and hear the stress contrast
Say it in this short phrase: She spoke fervently. Keep a strong FER and let vuhnt-lee move quickly, so the word tumbles off the beat rather than holding its own stress.
Hear the stress contrast: She spoke FERvently is correct, while She spoke ferVENTly moves the beat to the wrong syllable. Both use the same letters, but only FERvently, with the beat on the first syllable, is how dictionaries transcribe the word.
Feel the linking: spoke fervently runs together with no pause after spoke. The v of the second syllable starts as soon as the first vowel finishes, so keep your lips ready early.
Say the word on its own and let the beat land hard on FER: FER-vuhnt-lee. The last two syllables stay light and quick, with vuhnt-lee falling away quietly after the strong first beat. If the ending starts sounding as heavy as the start, tighten it back up: FER-vuhnt-lee, not fer-VUNT-lee.