Put on eye protection, along with work gloves and a dust mask.
Use a chipping hammer with a chisel bit to break up the concrete under the exterior door sill then pull out the rubble by hand. Vacuum out any remaining pieces with a shop vac for a clean surface.
Combine one part Portland cement with three parts masonry sand inside a wheelbarrow then mix up the dry ingredients with a paddle mixer. Wet the cement/sand mixture with a garden hose lightly then mix it again until it becomes the consistency of mud.
Pour concrete bonding agent into the exterior door sill area, allowing it to spread over the remaining concrete foundation. Immediately, tilt the wheelbarrow up, allowing the concrete to pour down onto the bonding agent before it dries.
Even and smooth out the wet concrete with a trowel then let it dry and harden for 24 hours before walking on the sill or using the door.