Asalumalykum how do we make our intentions sincere to Allah? Do we formulate words words in our thoughts that we’re doing this only for Allah?

No, you do not put it into words. The intention is in the heart, and pronouncing it or formulating it in phrases is an innovation, since the Prophet ﷺ and his Companions did not do it. Sincerity is simply that you intend Allah alone by the deed, and when a thought of showing off comes to you, push it away and say aʿudhu billahi min al-shaytan al-rajim, and do not overthink it.