Concentration Station
Interact negatively with the Sim you want your character to fall in love with, until she is your Sim's worst enemy. In this cheat, the enemy has to be of the opposite gender. Start fights with her and insult her until the Sim dislikes and eventually hates your Sim. Do this until the other Sim's relationship meter is in the red. Purchase the Concentration Station from the miscellaneous items tab and make potions until your Sim has created one that is red. Make your Sim drink the red potion, and his worst enemy will become his lover. She will be transported to his side, and her friendship and love meters will skyrocket.
Friends First
Ensure that your Sim and the Sim you want her to fall in love with are at least at 20 for daily friendship and 20 for lifetime friendship. If their relationship is not quite there, right-click on the other Sim and talk about interests, joke with him and give him friendly hugs until both meters reach at least 20. Make sure there is a blue smiley face next to the friendship meter for the opposite Sim.
Queen Vicanco Roses
Put the two Sims in a room with no doors. Remove doors by selecting them while the game is paused, then hitting "Ctrl" and "Delete" at the same time. Buy three tables and put three sets of Queen Vicanco Roses on top of them. Keep selecting the roses and chose "Give Flowers to Someone." Keep giving the other Sim flowers until the relationship bar reads that he is in love with your Sim.
Old-Fashioned Way
Talk it up with the Sim you want your character to fall in love with. Slowly test the waters with her; try giving her lots of friendly hugs and tell her some jokes. Once her friendship level has approached family friend status and the little smiley face appears next to her meter in the friendship tab, begin testing more gutsy moves. Try giving her romantic hugs and small pecks, dance with her and entertain her. Speed up time to level three to make this process go faster. Once the heart shows up above the opposite Sim's head and in the friendship meter, your Sim has found his true love.