You just have to click on the question - rectangular square that shows the question. These are displayed when you log in to the site.
Then the question opens, if there are not already two answers present, then you will see " KNOW the Answer? Add it Here."
Click in that rectangular area.
Go on typing your answer. Add pictures if it is needed. Create the pictures by yourself. Write original answers. It is appreciated that way.
after you complete, just click on the "Add your Answer" blue rectangular button below the typing area.
you have one hour to edit your answer further - in case you want to modify, already added text.