How do you write all possible selections of two letters that can be formed from the letters A, B, C, D, E, and F?
You fix one of the letters to be the first and then write out all possible second letters, assuming replacement is allowed. Do this for every letter.
For example, fix A to be the first letter. Then all the possible two letter words are AA, AB, AC, AD, AE and AF. And then do this for every letter.
A way to check we've written out all possibilities is to find the total no. of possibilities and check if we've met that.