I'm assuming you have already hard boiled the eggs and removed the shells. Cut the eggs in half long ways, remove the yolks and transfer to a bowl.
Add the yellow mustard and mayonnaise to egg yolks and mix with a fork until creamy. If it looks like you need a little more mayo or mustard, add until you end up with a creamy mixture.
Season with salt and pepper.
Assemble
The easiest way to assemble is to spoon the egg yolk mixture into the egg whites but if you want to get fancy, you can pipe the yolk mixture into the eggs using a pastry bag with a fancy tip. If you don't own a pastry bag, try a zip lock bag with one of the corners cut off. Works great!
To finish up, dust the deviled eggs with a little paprika and top with a parsley sprig.