Run a stiff wire brush over the initials in the concrete to remove debris and any loose pieces of the concrete.
Pour a small amount of concrete bonding liquid into a plastic bucket and brush the liquid into the initials on the concrete. A thin layer of the bonding liquid is sufficient.
Mix a patch of pre-mixed concrete with water in a plastic bucket. Strictly follow the ratio of concrete mix to water as outlined on the packaging.
Wait for the bonding liquid to dry for 12 to 15 minutes and scoop the wet concrete into the initials with a trowel.
Firmly pack the concrete into the initials and use the flat side of the trowel to smooth the wet concrete even with the surrounding concrete surface.
Wait 12 to 15 minutes while the concrete dries and run a damp broom over the patch. This will give the new concrete a bit of texture to help it blend in with the rest of the concrete.