I need help to understand how to work flowify on the circular surface, for example, I have a geometry in our case 3d letter A and I have to project this on a shield
I would suggest you go to Sketchucation and ask your question there. The author of the extension may already have a tutorial you can apply to this or will give you some guidance.
If you aren’t fixated on using Flowify, you could use Drape from the Sandbox Tools and then Joint Push/Pull.
Here is an earlier post about using Flowify on a sphere.
I would think that as long as you keep your element centered on the quad/grid, you will be able to cut a disc out after the Flowify procedure has been finished.