Remove the old caulk that is located around the shower or tub area and up / down in the corners. First use a utility knife with a new blade and try to separate the old caulk from the shower base or tub. After you cut into the old caulk, use a scraper and a hammer to gently remove caulk from shower base or tub. You also can use a straight edge razor to remove caulk residue from the tiles, tub or shower areas.
If you see black areas after caulk removal, this could be mold. You can try some bleach to kill mold located on the tiles, and shower or tub areas. Make sure you clean and remove the bleach from the area and let dry for 24 hours, before you apply new caulk. Never apply caulk after you installed grout. Let the grout dry for 24 hours before you apply new caulk.
CAUTION: Do not use ammonia on grout, caulk, and with bleach together. The combination of both these chemical will cause toxic fumes which is hazardous to your health.
You should use caulking where the tiles meet the shower base or tub and in all corners of the shower and tub area. Use good caulking that is made for kitchens and baths; not all caulk will apply well in the shower and tub area. Read the application directions for the brand that you use. I like to use caulk that you apply with a caulk gun. Cut the nozzle on an angel but do not cut a big hole. I would cut slightly bigger than the size of a medium long nail so the nail can fit into the nozzle to break the seal.
Place the caulking tip on the area you want to caulk and spread while moving rapidly from one side to the other. Don’t stop your movement even if you skip an area. You can go back and apply. When you’ve completed one side, spread caulk evenly from one side to another. Do not apply too much caulk at one-time, it is easier to add than to remove.
TIP: Depending on the brand of caulk you are using, you may spread the caulk either with your finger (without a glove) or a sponge with water. Check the brand of caulk to see if it is water base or not. Water base can be spread with your finger or sponge with water, otherwise you would have to use your finger to spread the caulk. And don’t use too much caulk, less is best!
You must wait for grout to completely dry before you apply any caulk. When you have completed all the areas with the newly applied caulk, make sure you allow at least 24 hours to dry before you shower or bathe.