This is very easy even of you don't know the rules very well yet :)
Turn on BLOCK DUPLICATES, SHOW INCORRECT and SHOW GUIDES. After this, just click on any number from the bottom of the board and some purple squares will appear on all the available spaces where the clicked number could be placed. Now, fill the 3x3 squares by placing a number only if you have just one purple spot available for a number in that house (3x3). Keep on filling the spots and after 5 very easy games, achievement unlocked :)