Not very difficult to do. I set up a new household with two adults; a boy and girl of course and set the relationship to HOUSEMATES.
I had their traits as follows: Flirty, Friendly, Lucky and Family Orientated.
Once the game finished loaded, I started flirting and then had a First Kiss (30g). After a bit of flirting, we had Woo Hoo [kind of obvious what this is :p] (40g).
Keep looking in the romance menu and after some more flirting there will be an option to "take things steady". This actually means being in a proper relationship and straight afterwards I proposed and got the achievement (50g).
This is the easiest and quickest way to get the achievement, and you will get others on the way towards it