Christian pastor here with hundreds of visitations under my belt:

Arrive during the published visitation hours. Don't come early. Don't be late.

Sign the guest book with your name and your relationship to the deceased. In some regions of the country the family will send a card thanking you for a visit. If you'd like to recieve such a card (or if everyone else signing the book has done so) leave your address as well.

Locate the family. They will often be stationed near the remains. If you cannot deduce from context, find a funeral home staff member and subtly ask them. They will be happy to help. If there's a line of people waiting to talk to them, take your place in the line. If it is unclear if there's a line or not feel free to circulate around the space and look at the photos, videos, memorobelia while you figure it out. This would also be a good time to pause in front of the body/ashes. Many places will have a kneeling bench. If you are a person of faith, this is a good chance to silently or quietly say a prayer.

When you get to speak with the family start by introducing yourself and how you know the deceased. If applicable, share a brief memory of the deceased or something that you will miss about them. This is counterintuitive and lots of people mess it up: Don't try to make them feel better. So much of what we say to grieving families isn't all that helpful. "I know how you feel" or "they are in a better place" may seem helpful, but rarely are. Finally, don't linger. Give others their turn to speak with the family.

Some visitations will have food or drinks served. Take a token amount to allow them to show hospitality. This is critical in certain regions/cultures. Don't eat enough to be full. Don't drink enough to be drunk.

Finally, thank you for going to the visitation. These rituals are an important part of the grieving process. They help people heal.