This will most likely be the most difficult stain for you to remove. My first advice would be for you to call a professional.
Step #1: Pick up, lift out, and remove as much of the mustard as you possibly can “without smearing” it into the carpet pile.
Step #2: Dampen the stain with water, and lightly blot out onto a paper towel. Repeat this process until all the mustard has been blotted into the paper towel.
Step #3: Dampen the stain with water, fold a few paper towels and lay it on top of the stain. Put a heavy object on top of the paper towel (like a brick) and let it stand until the next day.
Step #4: Apply a small quantity of cleaner to the stain. (To make the cleaner solution – mix 1/4 teaspoon of a hand dish washing detergent such as Dawn® or Joy® with a large cup of water) Use a blotting motion to work the cleaner into the affected area. If the stain is being removed continue applying detergent and blotting with a white paper towel until the stain is gone.
Step #5: If the stain is still there, flush with water to remove the cleaner, blot with paper towels and then let this stain rest until the next day. At this point the carpet fiber needs to rest and dry. You have done a great job in preparing for step #6 the next day.
Step #6: The next day, if the stain is still there: You are going to use a very common household product of 3% Hydrogen Peroxide. Moisten the stained area with the Hydrogen Peroxide (using a spray bottle) and let it stand for one hour. Repeat this one hour later if necessary. If any staining remains – Agitate, rough up the area with a paper towel, spray once again and then put a folded paper towel on the stain and a weight like a brick on top of the paper towel and let stand until the next day.
NOTE: Light will cause peroxide to change back to water so no rinsing is necessary.