how do you cook them.

For egg salad sandwiches, you hard boil the eggs. That means you boil them long enough that the yolk is cooked until it is hard, not runny. I cover the eggs with water and bring the water up to a boil. Then I take the pot off the heat, cover with a lid and let them stand for 10 minutes. Some people use a slow boil and boil them for 10 minutes. Put them in cold water right after the 10 minutes to stop them from cooking any more. Once they are cool, peel them and chop up for your recipe.

thank you very much.

Little tip for peeling hard boiled eggs: Hold the egg under water in a bowl. Crack the shell all over the place either on the bowl bottom or just squeeze gently with your fingers till it's a total crack job. Then the peel just slips off the egg and the egg is washed by the water. When you have all the eggs peeled, just drain off the water and it's easy to dispose of all those egg shells. No picking each little pieces of shell off an egg and risk of tearing it especially when making deviled eggs.

can you put mustard in egg salad.

Depends on the recipe. I usually put some in mine.

thank you guys very much.

I use a little brown mustard and dill pickle juice in mine.