This achievement requires that you have two Sims get married (not just propose marriage, as the description hints). Have two Sims constantly interact until they becomes friends, good friends, start dating, and then becomes partners. The best interaction for this is to "Romantic." At this point, the option to "Propose to Get Married" will show up. Clicking on this will bring up a menu where you purchase a ring. The more costly rings have a better chance of succeeding, so your best bet is to go for the Eternity Ring for 10 LP (100% success rate). Once engaged, continue the romantic interactions to fill the meter. The option to get married will come up when the bar is full. Select it to pop this achievement.