I need to know exactly how to put blade back on lawn mower.

Hi,
Not much info there! You will need the proper tools first. Then a piece of 2x4 to block the blade while you tighten.
If you will look at the very top post that has a label "Sticky" on it; it contains a great deal of this type of info (posted by other's than me) including one post devoted to this.
Peace,
Clarke