This achievement is really easy to do. The easiest way would be to play until you get an S. Along with this S, you want to have either an A, E, I, O, U or H. It's not often that you don't have a vowel, and S's are pretty common, so this should be pretty easy to prepare for. When it's your turn, find a word on the board that you can add an S to the end of to make it plural, then branching off of this S, make one of the following words:
As
Es
Is
Si
Sh
So
Os
Us
That'll be two words and will get you the achievement.