Make sure both Sims are in a romantic relationship
Increase the relationship bar enough to unlock romantic interactions
Have one Sim propose marriage using the Romantic interactions menu
Wait for the other Sim to accept the proposal
Plan a wedding event from the calendar or use the wedding event options
Invite the guests you want to attend
Choose a venue or host the wedding at home
During the wedding, select the wedding arch
Choose the option to get married
Complete the ceremony for the marriage to be official