You must first look at a person's info to see if they are interested in your gender. After establishing that they are, you must use expressions to progress their opinion bar towards Love. Look at their details to see what they like to see, get as gifts, and where they like to be. If you perform the liked actions within the liked area, the opinion bar will increase the fastest. When the bar passes the wedding ring icon, and as long as they have a heart above their heads, you can give them a ring to propose.
You must first look at a person's info to see if they are interested in your gender. After establishing that they are, you must interact with the person to take them from 'neutral' to 'friend'. This can be done by using the positive interaction button. Once you have moved a person's feelings towards you as far as possible at 'neutral,' the person will ask you to complete a quest for them. Do so, and they will become your friend. Continue positively interacting with the character until you are at the very extreme end of 'friend.' The person will then ask you to take them on a date. Do so. After dating the person, proposing marriage will become possible. Make sure you have a wedding ring ready, and then propose marriage.