These are the steps:
1. Gather enough lumber and gold.
2. Upgrade your house to level 3.
3. Have, to be safe, 3 hearts with the mayor.
4. Have 30 notes or more. You cannot get married with less than 30 notes. [Is it like a maturity thing? O.O]
5. Have 8 to 10 hearts with your future partner.
6. Theodore will knock on your door the next day all this is done! :D
"Hello my waffle minions."
"YOUR minions?! >;o"
"Yep, now pass me the syrup."
"Never."