Summarize this article:

A polymer grout additive increases the grout's durability, but read the grout's label first, as it may already contain an additive. If it does not, you may purchase an additive and follow the instructions on its label while mixing, replacing some or all of the water as directed. Otherwise, the process is the same as described below. Epoxy grout products typically consist of two or three components, and the mixing proportions between these vary depending on the brand. For more traditional grout products, the steps below should work, but check the label first in case of unusual instructions. You'll need an empty bucket, a container of water, and a sponge. Find a pointer trowel, margin trowel, or a mixer paddle drill bit to use to mix the grout and apply it. Finally, pull on a pair of gloves. Measure out all of the grout powder required for the quantity of grout you’re producing and pour it into the bucket. Check the grout label to find out how much water you need for the amount of space you'll be covering. Pour ¾ of the amount of water required in with the grout powder. If you have a large area to cover, consider mixing half the grout at a time, so the grout in the bucket doesn't dry out before you're done. Use your trowel to mix the powder into the water, until you've made a thick paste with no dry lumps. Tilt the bucket toward you slightly, rolling it on its edge as you scrape off any dry grout from the sides. If you have a mixing drill and attached grout paddle, you can use that instead. Stay below 150 rpm to avoid weakening the grout with excess air bubbles. Add water one squeeze at a time from a sponge, mixing it thoroughly into the grout. Aim for a smooth "peanut butter" consistency, with no lumps. If the grout gets watery, pour in a little bit more powder. Allow the grout to "slake," or strengthen through chemical reactions. Leave your trowel on newspaper or other surface to prevent a mess. Briefly mix together the grout again, as it will become slightly stiff during slaking. Use immediately, as most grout sets within 30–60 minutes. If the grout has already hardened, you'll need to discard it and make a new batch. Adding more water after slaking will not be effective.
Consider an additive. Follow label instructions for epoxy grout. Gather materials. Add the grout powder to the bucket. Add ¾ of the total water needed. Mix the grout with a trowel. Squeeze in more water with the sponge. Leave the grout alone for 5–10 minutes. Remix and apply.