Hammer two 10d nails into the opposite ends of one foundation wall. Leave about 1 1/2 inches of the nail sticking up.
Tie a string taut between the two nails, and use a transit to level the string as close to the top of the concrete wall as possible without touching it in any spot. A tiny gap of about 1/16 inch is sufficient.
Mix water and high-strength mortar mix in a wheelbarrow. Follow the mixing directions on the mortar mix, and stir the wet mortar with a garden hoe or a small shovel.
Scoop up the wet mortar with a mason’s trowel and deposit it on top of the concrete wall at one side.
Spread and tamp the wet mortar flat against the concrete until it’s level with the string.
Continue using the string as the guide and work your way down the wall, adding mortar and leveling it with the mason’s trowel as you go.
Repeat with the other concrete walls, but leave the original string in place. Just drive a new nail at the end of the next wall and tie a new string between the two nails, pulling it taut and leveling it with the transit each time.
Let the mortar dry before removing the nails and setting the sill plate.