It may look easy when a professional is repointing brick, but there are some things to be very careful about.
If you are using a grinder to remove the old mortar, be careful to avoid cutting into the brick. It is easy enough to slide up or down hill with the grinder, and even easier to mess it up when trying to cut out the vertical mortar.
Even after passing with the grinder, you need to take a mortar chisel and remove where the round shape of the grinder could not go.
Most workers who work on historical buildings do not use grinders as it is just too easy to do damage.
When you work with the new mortar, first soak your board in water. This will prevent the wood from drawing too much water out of the mortar and changing your mix. Mortar is too dry if a fist full of it just crumbles when you open your hand. It is too wet if it just squeezes away through your fingers. It is just right when it stays as a firm ball when you open your hand.
For important limitations on working with mortar, click here.