Disclaimer: I have never done this myself - only read about it.
Resetting. I think that's the key. There are different ways to get the villager the item you want (that I can't remember from the post I read). Then you can save I front of their house and keep resetting until you like its location.
I recommend doing a forum or tumblr or google search on the following: acnl villager interior design, acnl villager house, acnl villager house decorating.
Hopefully those searches will turn up some actual and useful guides.
A good time to try is when a villager is in their house and have asked for a replacement.
Hopefully they have an item that is the same size as the replacement one. And that it's in the same spot.
I customized a table and made the mistake of mailing it to the villagers. I forgot that she had 2 fish on the table so it was a waste.
Sometimes you have to try a lot of times to get them to put the item where you want. Using other players with good friendship levels helps. I've been using my other towns mayor who has a good friendship. She writes a letter and puts an item in.
They'll wear and put the items in the house from her.
usually they replace like items, ex. a bed with a bed, a dresser replaced with a dresser, etc. or items of the same size tend to be replaced/swapped.
i also strengthen what stitchmaker says, it's a ton better to try to obtain furniture replacement requests when they're in their home. that way, you can come back with your item and give it to them, leave, go back in and see where they replaced it. you can then restart your game if they don't use it or replace something that you wanted them to keep.
mailing and forcing them to buy it from retail can work, but it's way more risky. villagers oftentimes have items that they won't get rid of, so if they refuse to get rid of a 2 space item and you send them a 2 space item, it's highly probable that they won't replace it.
this is all based off of my own personal experience, so i could be wrong. these have been consistent in my endeavours though.
The best time is when they ask you to help them replace a specific thing. That way, you know the item will go right back where it should. That is why you should refrain from removing a piece of clothing or anything else that is in the spot you want to get back to original when they have invited you over. If you do, the spot will be empty and it'll be luck getting the item back there. It takes patience. Erik has had a shirt on his floor for a long time and I've been waiting a long time for him to ask for me to replace it.
As mentioned above, when they just ask for something in general, you should wait until they are in their house and give it to them. If you don't like where they put it, quit without saving. Beau had a robo-lamp where his doghouse was supposed to be, and it was casting an ugly green glow over his room. He asked for general furniture, and I saved and quit until he was in his house. Then, I gave him his doghouse. About ten times, he put it in the wrong place. I just quit without saving until he got it right. It's tedious like plot resetting, because sometimes he isn't in his house. But, patience paid off.
