Apply the Mortar
Use pre-mixed mortar to apply the scratch coat. When mixing mortar, always start with less water than you need because you can always add more.
When the mortar is the consistency of frosting, use a rectangular finishing trowel and a London style mason's trowel to apply generously and thoroughly over the lathe. Start on the inside and pull in from the corners. After applying the scratch coat, rough the surface up with a whiskbroom. This will allow a better bond for the stone. Allow at least two hours for the scratch coat to dry. Utilize this time to lay out the stones. Be sure to vary the size and style of the pieces for a more natural look.