as salam alaikum
sincerity stems from the heart and it is not something conventional that can be obtained by merely verbal expressions or symbolic acts. The servants should first look at the greatness of Allah, the bounties he has received from Him, and the gravity of his sins. Then he should see where he stands in front of Him and where he is aiming vis-á-vis His will. So he will feel remorseful for his past errors and he will intend to rectify himself to please Allah.
There is no specific actions to be performed to gain sincerity in asking forgiveness from Allah, rather actions would follow the awareness of the heart and the penitent soul who realized his mistakes and to Who he has disobeyed.
With prayers for your success.