I do not wash my mushrooms in water. Mushrooms are like sponges and soak water up, often making them mushy and tough.
I use a paper towel wrapped around my finger and quickly and lightly wipe the dirt off of each mushroom. If the mushrooms are very dirty I use a lightly damped paper towel. You can also use a mushroom brush.
Slice the stem to make shorter.
After the mushrooms are clean and the stem cut, slice them any thickness you want or need for a particular recipe.
If you still need help in preping mushrooms check out this description and video: Prep Mushrooms.