Start an original game with a local account as your opponent. A good way to do this like others have mentioned is to make a small word that you can turn into a larger word. I used this page to list all the 9 letter words in the scrabble dictionaryhttp://homepage.ntlworld.com/adam.bozon/Scrabble9.htm
If you have the tiles for a 3-4 letter word just run a search on the page linked above for that word and you can scan through all the words you can form around it. for example I spelled the word "one" a simple search shows there are a lot of 9 letter words that contain "one" (ABANDONE
...the list goes on for a while) since I already had an 'a', 'p', 'h' in my tiles I decided I would try and go for the word "EARPHONE
simply discard any letters from your hand by marking them with x and discard them with the Y button to end your turn, if you get any of the letters you need to finish the word keep those and discard the rest. the last letter I needed was R (also one thing to note if you are playing with a 2nd controller which I suggest, make sure they are not holding some of the letters you need. i.e. my 2nd account was holding an R and 2 S's so I made them discard them so I had a better chance at drawing them from the pile)
again most of the other solutions have touched on this but its nice to have all the options in front of you.
Keep in mind that you have to place at least 5 tiles so at max you can start with a 4 letter word unless you want to try for a 10+ letter word but lets not push our luck.
A fun one is to try and spell "word" and go from there to get PASSWORD
PLAYS ...you get the Idea, and hey is the word "word" starting to sound really funny when you say it now? either way after you have this achievement you will be a WORD