Once you have finished laying your brick, remove the drop cloth and clean up your working area. Tape a plastic drop cloth to the floor -- grouting can be a very messy process.
Prepare a premix bag of mortar according to the manufacturer’s instructions. Use a trowel to stir water into the mix until the consistency is close to that of peanut butter. Mortar can be harsh on your hands, so it's a good idea to wear gloves.
If you have chosen to set your sills with stone veneer, they can be set with the mortar mix. Set the mortar in the sill with the trowel, keeping it away from the brick and wood -- mortar is difficult to clean up so it pays to be careful. Set the sills and check them for level, side-to-side and front-to-back.
Fold over the end of the baker's bag to prevent it from leaking with you fill it, and then use a trowel to fill the bag with mortar (Image 1). Test the flow of the material before you start to make sure it will come out at a controllable consistency.Begin grouting by running the baker’s bag along the joints and slowly squeezing the mortar into the gaps (Image 2). Work slowly and carefully to prevent mortar from getting on the brick face, as it's difficult to clean.