Remove the grout from around the cracked tile. By far, my preferred method of removing the grout is to use an oscillating multi-tool and grout blade. You can use a sturdy box cutter, but it will take much longer.
Grout the tile. Use a grout float to push the grout mixture into the lines and then wipe away excess grout. Make sure to push and wipe in multiple directions to completely fill the gap.
After about 10-15 minutes, use a damp grout sponge to clean excess grout and make the lines smooth. Cleaning as much grout as possible will make haze removal easier later.
Haze removal. Wait at least 24-48 hours and wipe several times with a clean, damp sponge to remove any haze. Use a water bucket to rinse the sponge several times. It may take a few cleanings to remove all of the haze and more will show up as it dries.
Sealing grout. I do recommend sealing all new grout for longevity and maintenance. Even those with pre-treatments in them will last better with an outer sealant.
Caulk showers and tub surrounds as needed. I always caulk after the sealant is dried.