Unlike a ceramic or porcelain tile floor it s not just the grout lines where sealing is critical but the entire surface.
Applying sealer to marble tile pre grout.
The same process used to seal marble floors can also be used to keep a marble countertop sealed against stains.
Use an impregnating sealer.
Then wipe off all excess until completely dry.
I would seal the tile before grouting.
For the walls you ll have to spray on the sealer and wipe around with a paint brush or sponge to keep the tile and grout wet with sealer long enough to soak in.
Marble sometimes gets circulary etches scrathed into its polished face with sanded grout.
Step 3 application.
The spray on variety is the most common although some types can be applied with a sponge or brush.
Then grout using non sanded grout.
Apply your sealer to the lines and joints of the grout.
Then wait 3 days minimum after grouting giving time for grout to cure and then seal the grout.
Apply the sealer to the carrara marble tile and grout by creating a film of sealer.
Let it soak in for 2 5 minutes.
You can apply another coat of sealer on the tile too.
To protect your tile properly seal it a second time after the grout cures.