Say /prəˈnaʊnsɪŋ/. Both British and American English place primary stress on the second syllable: pro-NOUN-cing. Keep the first syllable weak, pruh, not proh.
Start with a relaxed pruh, lips neutral.
Stress now, opening the mouth wide as in cow, ending in n.
Finish lightly with sing, keeping the s soft.
Compare noun with pronouncing: the now syllable carries the stress, so avoid saying PRO-nouncing.
I keep pronouncing it wrong.