Instructions
Upgrade your house two times before your propose. Upgrading your house costs 10,000 gold pieces the first time and 50,000 gold pieces to upgrade the second. This will expand your house and yard, giving you more rooms and farmland.
Go to the general store and buy the double bed. Install this in your home. You will need a double bed so you and your wife can sleep in the same bed.
Talk to the girl every day. Bring her gifts and also attend each of her "heart events" to get her to like you.
Watch the heart meter of your girl. The meter starts at blue and as her interest in you increases, changes to orange and then red. Orange means she is growing fond of you while red means she is in love.
Purchase the Blue Feather when your girl has an "orange" heart meter. The Blue Feather serves as the "proposal gift" that lets her know you want to marry. Keep talking to her until she has a red heart. Give her the Blue Feather to propose. She will accept, triggering the marriage animation and event.