To create an archway, click on the Archway tool, select the desired archway and place the archway on an existing wall. Most doors have archways designed to compliment them, so you may want to consider that when choosing an archway.
Available archways range from the exceedingly plain to the very ornate and come in either one or two tile versions. Note that the archways take up two tiles for the one-tile version and three tiles for the two-tile version, so you need to have adequate free space in your room.
Did you know
Keep your Sims traffic flowing smoothly by placing additional archways to rooms. This makes sure your Sims won't bar each other's way.