Step one: Choose a position (1 through 4, in this case), and swap the letters at that position. The result of your swap must be a pair of valid words.
With reference to the example, in this case we could choose the 4th position and swap to obtain:
Step two: Find an anagram of both of your new words.
With reference to the example, we have:
That's it. First swap, then scramble. Have fun.
The three-letter example is suitable for kids, if you first carefully explain the steps using FIRM/SOFT as an example.