How the Iowa Caucuses Work, Step by Step

To people accustomed to voting in primaries by walking into a polling site, privately completing a ballot and walking out — which is to say, most of the country — caucuses may seem bewildering.

They certainly did to Mary Doyle when she went to her first caucus more than 20 years ago, after moving to Iowa from Illinois. “I had no clue what was going on,” she said. “It was all brand-new. These people get up and talk, and everybody’s talking, and I didn’t know anybody.”

Like many Iowans, Ms. Doyle — who will be a precinct captain for former President Donald J. Trump at a caucus site in western Des Moines on Monday night — came to love the caucus process, with its open expression of candidate preferences and attempts at last-minute persuasion.

Caucuses also have their critics, who note that the process makes voting inaccessible for some people and tends to lead to lower turnout than primaries.

But love them or hate them, they are still happening. At 7 p.m. Central time, caucusgoers will gather — if they are not deterred by the bitter cold — in schools, community centers and other sites across the state.

Here is a step-by-step guide to how the caucuses will work.

Voters will arrive at their designated caucus site and sign in. Workers will check each person’s ID and voter registration. While only registered Republicans can participate, people are allowed to register or change their party affiliation on the spot.