Instructions
Click on the Sim you want to fall in love with your current Sim. Click "Talk" from the menu of options. This will get the initial friendship started.
Select one of the other options from the menu to take the relationship further. Options include, "Tell a joke," "Compliment," "Gossip," "Hang Out" and "Hug."
Invite the Sim over to your Sim's house if they don't live together. Cook them a meal and talk while you're eating. Talking with the Sim is the easiest way to increase your relationship score.
Utilize the hot tub with the Sim once your score has reach friendship status (50-point relationship score.) While in the hot tub, you can use many of the same interactions; such as talk, snuggle, kiss, make out and WooHoo. You can also splash each other with water, which usually gets a positive reaction from other Sims.
Use the options for "Hug," "Kiss," "Caress," "Serenade," "Flirt" and "Make Out" once the relationship score has reached 70. Using these actions repeatedly will increase the score up to the required 100.