Mixing
Mixing the grout is the first step to ensuring there are no pinholes. The reason the instructions on the side of the container tell you to mix the grout by hand or using a low-speed drill is because mixing the grout too quickly whips air bubbles into the mixture. Once the grout begins to set up in the joints, these air bubbles can move to the top and burst, leaving pinholes behind.
Slaking
A common mistake is for people to ignore the slaking process of grout mixing. This is a period of time after the initial mixing where the grout is allowed to sit for 10 or 15 minutes to allow all of the polymers, pigments and additives in the grout mixture to absorb the water and soak everything in. After the grout has slaked for 10 minutes or so, you can remix it and proceed to install it. Grouting without waiting for the grout to slake creates an imperfect mixture that can cure and leave voids and holes behind.
Grouting
The installation of the grout during the grouting process is also important. A grout float needs to be used to force the grout into the joint from multiple directions. This ensures that the grout not only fills the joints but also any potential voids left under the edges of the tile during the installation. If these voids are not filled with grout, they will create voids that crack out over time and leave holes in the surface of the grout.
Cleaning
The water to dry mix ratio is important to the strength of the grout, and if you use too much water when you are cleaning the grout from the face of the tile during the grouting process, you can ruin the strength of the mix. This leads to watered-down grout that will crumble within weeks and leave behind voids that mar the surface of the grout.