Hi everyone,
Some of the grout are missing from the tiles next to the bathtub.

What step do we need to take? Any product recommendations?
Should use some cleaning product to give it a clean first?
Its our first time doing this, don't have any idea where to start.
(We have a Ultra White Grout Dunlop that we bought in Bunnings long time ago.
Haven't used any of that before.)
Thank you for any advice..!