First, you'll want to acquire complete sets of furniture. Use these lists to help you:
Furniture:
http://www.thonky.com/animal-crossing-new-leaf/list-of-furniture/
Carpets and Wallpapers:
http://www.thonky.com/animal-crossing-new-leaf/list-of-wallpapers-carpets/
If you can get an entire set, such as the Lovely set, you will get a points bonus. If you include the wallpaper and carpet from the same set, you get an even bigger bonus.
You also get smaller bonuses by matching themes and styles. Those are shown on the furniture list as well. And if 90% of the furniture items in a single room are the same color, you get a points bonus based on how many items are in the room.
You should decorate in this order:
1) Complete your sets
2) If items in a set are the same color (such as the Lovely set or Green set), then get other furniture items with the same color.
3) If items in a set aren't the same color (such as the Patchwork set), then work on Feng Shui. Red to the east, yellow to the west, green to the south. However, if your set items are the same color, it might be wise to ignore Feng Shui in order to get the bigger points bonus from matching colors.
4) After you decide whether or not to match colors, grab some miscellaneous items from the same theme or style (in that order). Just remember the 90% rule for color matching points.
After you get pay off your loan:
Lyle will start grading your house differently. You'll need to confine your entire house to a single theme, such as the Fairy Tale theme. Make sure that each room only has your specific theme's furniture in it, as this is the only furniture that will be counted. You will then get a points bonus for each room that is 90% that theme, based on the number of
exterior house parts that are that theme. For example, if you have 5 exterior Fairy Tale parts, then your points bonus for each Fairy Tale themed room will be multiplied by 5.
To reach the 1,100,000 mark, you'll need to have all of your rooms (excluding the basement) littered with furniture from the same theme, while having all of your exterior parts be from the same theme as well.